Get the best quality irons your can afford.... and quality means maximum "feel".

For the golfer who can break 95, but is not ready for a pure muscle-back, the Ralph Maltby MTF carbon steel forged cavity back will give you a higher, more floating approach shot with some stopping power. VERY nice feel. Provides considerable forgiveness, but is still quite workable.   Not cheap at about $60 per club with steel shafts. With reasonable composite shafts, figure around $65 each, up to $100 or more with high-end shafts.. 

The unforgiving Wishon 550M forged blade. Very nice stuff.  For the really good golfer who practices a lot and makes accurate contact almost all the time. Note the minimal heel and toe weighting, and the extra mass behind the sweet spot. Super workability, trajectory, backspin, incredible feel, and your ball floats down onto the green like a snowflake and stops right there. Though these are not cheap, you will spend far less than for an off-the-rack set of similar quality, and they are life-time clubs.  Will need tuning every couple of years.  About $60 per club with steel shafts.  

IRONS
Price-  Figure around $25-$65 per iron with steel shafts, and add $10 to $80 with glass shafts. See Quality

Set Makeup-  If you cannot drive the ball over 260 yards, don't carry anything longer than the 5 iron.  In most cases, I recommend getting the fairway wedges offered with your chosen iron set, up to the GW or AW, but not the sand wedge, and you probably still need an additional "trouble" wedge. See Set Makeup

Length- There is a formula for the shaft length of your irons, but we can fudge it up a bit for more distance, or down a bit for accuracy, depending on your game improvement needs.  Irons are not supposed to be distance clubs; they are for hitting the green, so it is accuracy and consistency that you want, within an acceptable distance range. Backspin is also an issue, but only if you are capable of hitting down on the ball accurately. For better golfers, trajectory is also important

For mid-handicap men of average height, your 7-iron should be spec'd to carry 140-150 yards.  

"Game Improvement" Clubs - a number of design features are lumped under the phrase "game improvement", so its helpful to understand what that means.  

There are 20 or 30 ways you can hit a ball incorrectly.... off-center laterally or vertically, topping it, skulling it, hitting the ground behind it, wrong swing path, open or closed face, hitting with a descending or ascending swing path, etc...  Some of these defects are swing path issues, and some are club-to-ball contact issues.

While a properly fit club can help with some of the swing-path issues, the design of the clubhead can only address the contact issues.

A number of clubhead design features attempt to compensate for bad contact, and that's what is mean by a "game improvement" design.  But "game improvement" design can help correct only 5 of these defects, and they only make a slight improvement in the result of a slightly bad contact.  They DON'T DO ANYTHING to improve a bad swing. A really bad hit still produces a really bad result.

On the flip side, on those occasions when you make great contact, these same features take something OFF the result.  A well-struck game improvement clubhead produces less fell, backspin and workability than a well-struck muscleback 

Head Shape- With a few exceptions (the "muscle-back" blade designs intended for better golfers), most iron head designs are perimeter weighted.  The difference is in where on the perimeter the weight is concentrated.  All clubs have most of the weight towards the bottom, so the question of weight-distribution is really "how much". Heel-and-toe weighted clubs are slightly more accurate, sole-weighted clubs will help get the ball up in the air, and top-line weighted clubs are more forgiving of vertically off-center hits. The current vogue is "undercut" cavity designs or "power bands", where the extra weight is placed back from the face.  Whether there is real advantage to this remains to be seen.. it isn't very far back.

The trade-off with perimeter weighting, is that it will "correct" your mis-aligned hits even if you intended to hit it that way.  So they tend to be less "workable" and will not take a draw or fade as well as a less forgiving club.

Offset- Offset refers to the small fraction of an inch that the face of the club is set back from the line of the shaft. High offset is intended to reduce slicing. Clubs for low-handicappers have zero offset, while hacker clubs have as much as a quarter inch. Offset slightly reduces your tendency to hit with an open face, thus giving you a slightly more accurate average shot. 

The theory is that a lot of offset means your hands will be a bit further along in the swing when the face contacts the ball.  Therefore (the theory goes) your wrists will be somewhat more rotated, meaning a less-open face. 

Again, there is a tradeoff... less workability, lower trajectory, and loss of feel.  And for structural reasons, high offset clubs are made out of 17-4 stainless, the klunkiest of the popular golf club materials.

Bounce- Bounce is the amount that the bottom of the club sticks down below the bottom of the face. Oddly, its measured in degrees.  Less skilled golfers want some bounce, so that the club doesn't dig into the dirt so quickly when they hit a little fat.  But not too much, or the face will make bad contact when they hit a little thin. 3 degrees is about normal on a fairway iron for average golfers.

Rocker Radius- Rocker is the side-to-side curve of the sole (bottom) of an iron. Clubs for better players are flatter on the bottom to produce better contact and keep the weight lower.  But less precise players would frequently dig those pointy extremities into the dirt, so they need more rounded soles. Overly flat soles are also a problem on side-hill lies

Grind Radius- The back edge of the sole of fairway irons (and wedges) needs to be "radiused" (have the edge softened) to avoid it catching on the grass on the takeaway.  Better players prefer somewhat less radius, again keeping a little more weight lower.

Material- While a lot of other design elements determine how each individual swing moves the ball, the material from which  your iron clubheads are made does nothing for a swing, good or bad. But it is important in improving your average result.  The material determines, more than anything else, the quality of the feedback each impact gives you.  This in turn allows you to improve the quality of your future swings.

Most irons (including premium brand-name off-the-shelf clubs) are made from cast 431 or 17-4 stainless steel, which provide a very durable and cosmetically nice club at a very affordable price, but have a slightly "clunky" feel.  304 stainless is  popular in the lower-end market for its better feel, but it is softer, less durable, and more troublesome. Hi-tech 455 stainless is also used increasingly in a lot of mid-range clubs, because it  requires less expensive handling at the factory.  

"Hot-forged" stainless steel clubs (meaning they are cast into the desired shape and then heated and hammered into higher density) are also becoming popular amongst some low and mid-range brands trying to upgrade a bit..  It is not possible to truly forge stainless steel, but hot-forging adds a little density (not hardness) to the face.

For superior density and feel, nobody has improved on forged carbon steel, a block of raw iron containing a small percentage of microscopic carbon flakes, simply pounded into the desired shape. Most very good golfers prefer them.  But forged  clubs are expensive ... about two to five times what stainless clubs will cost... and require occasional tuning.  

Ask me if you'd like to hit a forged club sometime; you'll be impressed.  For balance and feel, there is simply nothing better.

But like I said, a better material will not give you a better shot right off the bat.  What it does is allow you to improve your swing over time.

Loft and Lie- The "standard" 5-iron loft is 27 degrees, with a lie of 60 degrees. A 6 iron is 30/61, ...3 degrees more loft and 1 degree more lie.  (incidentally, those are market-driven specs... read about "loft/length creep".) These changes get bigger as the numbers go higher. Some designs go "stronger" on the loft, and some go higher, so it is possible to build a set that meets your vertical game plan.

 If we have to play with the shaft length to get the right distance from your irons, then the lie may need adjusting. Also, if you are built with proportionately longer or shorter limbs than the ergonomic norm, the lie must be adjusted.  Finally, if you prefer a softer or stiffer shaft, or have a significantly faster or slower swing speed, this will affect your lie requirements, because your shaft bows outwards at the bottom of your swing, tipping the toe down.  

All that having been said, standard lie is just fine for most golfers, and fine-tuning an off-the-shelf set would not be a good value for them. Good golfers should be concerned about lie angles, because their typical accuracy is close enough that a canted head will throw them noticeably off. 

Manufacturing standards are not what you might expect, so many heads within a set vary up to a full degree, just by accident.  Even if it was right coming out of the factory, it will change with time and repeated impacts. You should have your lofts checked and tuned now and then. Clubs made of 17-4 stainless are just about impervious to this kind of accidental de-tuning-over-time, and really  can't be adjusted effectively anyhow, so there's not point in having them tuned... you are stuck with what you got out of the factory.

Ladies Irons-
Most manufactures will just change the paint job on a clubhead and label it a "ladies" set.  Same weight, same loft as their men's sets, though the price will sometimes be a bit lower. However, a few designers do offer slightly lighter, higher lofted irons for women, which make sense for most women golfers. On the other hand, some designers insist that ladies need heavier club heads, to compensate for the shorter shaft length.

Shafts- Stiff, full-weight, metal shafts are technically right for most people, are most consistent, and also least expensive.  But "feel" is critical to iron shots, and if composite shafts feel better to you, then you should use composite shafts. Lightweight shafts may work better for less physical golfers, and shock-absorbing shafts may be a medical necessity for golfers with joint problems. Unfortunately, they cost more, sometimes twice the cost of steel-shafted irons. Even worse, if you use fiberglass shafts, you should use good ones, and that gets pricey.

The stiffness of the shaft has no effect on trajectory in an iron. Its true that a softer shaft does allow the club to toe-down more at impact (see shaft bowing), but that is only a problem in extreme cases or for very good golfers.  In most cases, the shaft stiffness of your irons is a matter of preference, to produce the best feel for you.

Swingweight and Balance- Swingweight is one measure of the balance of a club; that is, how heavy it feels when you swing it.  "D2" is the traditional norm, but there is no "right", and many struggling golfers benefit from a heavier feel..  We can build a set that feels most comfortable to you, and encourages proper timing.. The only real standard for swingweight, is that all the clubs in your set feel to you as if they balance the same.

Some designers use a more scientific approach to balance, basing it on the dynamic moment of inertia about the length of the club, rather than static swingweight.  That's certainly a more modern approach to the "balance" problem, but I don't think even the most ardent supporter would claim that they have reached the final definition of "balance" in a golf club.  Balance is based on feel, but remains an intangible, and the only reliable way to tell if a club balances properly for you is to swing is a few times.  

If you have a "favorite" club, take it to your clubfitter so he can try to adjust the rest of your clubs to a balanced feel similar to it.