I am almost exactly the same size as you, and similarly swithed to shaped skis late in the game...where do you ski?
From the sound of your skiing preferences, I think the Head im78 171cm. would be ideal. It has more sidecut and a tighter turning radius than many other skis in its class, but somehow it doesn't suffer from the usual downsides of such a ski. In other words it still performs well for medium to long turns and is relatively stable (due to a stout race-like construction). Others would disagee, but I personally wouldn't even bother demoing a bunch of skis at this point. That can get expensive and confusing when you are first starting out n the world of modern skis, plus without some experience and some skill developed in modern carving, you won't be able to be as discerning in the demo process. There are a handful of models that you can't really go wrong with as long as you get a decent price on them, and the im78 is one of them. Once you get some time under your belt on such a ski, you can get more specific about what other skis you need for specific situations to add (or replace) to your quiver.
IMO, there's no point to getting skis that are purposely straighter in shape just to ease the transition to shaped skis..I think a forgiving but versatile and high performing "skinny" midfat with a good amount of sidecut would be the way to go; that is if your interested in evolving your technique to more modern carving style. Anything much skinnier than the im78 would likely not be wide enough for the kind of skiing you are describing (40/40/20), and anything much wider would be more challenging (unnecessarily so) to learn carving on and slower edge to edge.
The Recons are, or were considered a similarly universal, "can't go too wrong with" choice. It is similar to the im78 in that it is a stout but forgiving ski with metal in it, that is versatile and forgiving as well. Personally, I think you'd find the im78's to be a bit better at short radius turns b/c of its sidecut, and while it is also a pretty damp ski, it is less "dead" feeling than the Recons (and other Apaches).
In terms of length, I 'd recommend the 171's or thereabouts. And I would highly recommend mounting Head/Fischer/Tyrolia Railflex bindings on the im78's. The added leverage of the plate will greatly facilitate your transition to carving techniques, and the ability to experiment with fore/aft binding position is perfect for someone trying to figure out his/her "tastes" and preferences in modern skiing technique and equipment.
I personally tried many similar models until settling on the im78's...Nordica Nitrous (no metal=not as stable), Recons (not as good at short turns), Atomic Nomad Blackeye (too stiff for my weight, only good when skied aggressively), Fischer Watea 78 (too light and unstable in crud), Volkl AC 30 (too hard snow biased and stiff). The Elans and Blizzards are supposed to be excellent as well, but I never tried them. I doubt they would be better than the im78's though for those short radius turns you speak of.
So I would personally pick up some used im78 171's with Railflex bindings (or Recons if you really are loyal to K2) for, say 300/350$ or less (In fact I got mine for not much more new on clearance), and spend at least 7+ days on them. Spend the money you saved from not demoing a half dozen skis on some lessons and lift tickets...After that, you should be able to figure out if you need something fatter, skinnier, longer, more or less sidecut, more or less metal, whether you like being mounted a bit forward or backward on the ski, and go from there, now with enough carving skills under your belt to actually be really discerning when you start demoing.
Edited by monologuist - 3/17/2009 at 01:24 am