Welcome kikobb.? This forum is just a bunch of regular skiers, so you may want to also send a request to Peter from the pay site to get a more professional response.? I'll share my thoughts, but you have a unique set of medical problems to overcome.
1. Shaped skis offer the possibility of skiing with a lot less strain on the knees.? Because of the shape of the skis, they will turn just by being tipped. The bad news is that shaped skis can seem unstable to someone new to them because they tend to wiggle back and forth a bit when they are flat on the snow.? Many actually work better by being skied a little bit on edge all of the time.
2. Ski brand probably doesn't matter too much.? You should consider renting at Aspen (because they'll have pretty good quality skis and probably well tuned).? Then you could see about demoing a few skis after a couple of days of getting your feet on the snow.? As far as the icons on RealSkiers, you want to make sure that the ski has the green skier icon (and probably that it doesn't have the black skier icon).? If you read the explanations, Peter's colors are unrelated to trail classifications, and skis with the black icon are usually a bit stiffer and less forgiving.? You probably want an all-mountain ski, length around your nose so probably mid-upper 170's, and waist width in the low 70's with a turning radius around 15.? Skis with enough side-cut to have shorter turning radius will probably also feel twitchier to you.? You also want a ski with a high "smooth drift" number since that facilitates adapting old technique to shaped skis.
3. Look at the Fischer Progessor 8+ as an example, especially looking for review terminology of "excellent for anyone either learning to carve or working on building up current carving skills".? And then compare that review to the Progressor 9 and 10 which are both excellent skis but higher than I would suggest for you. The Kastle LX72 (Groomer Cruiser of the Year) is also a good choice, but Kastle's are expensive.? Interestingly, I saw lots of rental Kastles at Aspen last year, so consider that option.? K2 Rictor is a new model with a bit of rocker, but good reviews. Blizzard Magnum 7.4.? I'm not a fan of Volkl or Atomic, but that's just a personal preference, but based merely on the RealSkiers reviews you could look at the Volkl AC20 or AC3 Motion.
4. Boots are the most important part.? You should buy boots even if you choose to rent skis and you should have them done by a good boot fitter or a knowledgeable boot shop.? In general, most ski shops will fit you with boots that are plenty comfortable in the store and wind up being too large after a years use. Do a Google search for America's Best Bootfitters. What they should do is something like the following.? Take the liner out of the boot, and with you standing in the plastic boot and toes just barely touching the front of the boot you should have 1/2-3/4" behind the heel.? This is called "shell fitting" and tell a lot more than the "try the boot on and see what you think" approach from most ski ships.? With the liners back in this will feel pretty snug intially; lean hard against the tongue to see how they will eventually feel. Walking around the store in ski boots has little bearing on reality, it tends to push your feet forward in the boot, exactly the opposite of the forces when skiing. In addition to length, boots also vary by "last width". Racier boots (or boots for people with narrow foot) will have lasts of 96-100 mm while more common boots will have lasts of 103-104mm.? When you are standing in the plastic shell get a feel for how much space there is on the sides.? You should not be touching, but it shouldn't be much more than 1/4" or so (I know, that's hard to know). Also feel if the sides of the boot seem to match your foot shape.? With a good fit, expect that the shop will have to grind out a few places (for me, this is at the joint of the little toe and also the big toe).