You are right, ammo availability could rule out the 7-30 Waters. It seems to be a regional thing so you'd have to do your test or talk to people in your area that shoot it. Many or most people end up reloading for specialty pistols eventually anyhow. The returns in performance and accuracy in these pistols are of much greater potential than factory versus hand loads in most rifles. Other benifits from hand loading include reduced muzzle blast of rifle calibers in short barrels by using slightly faster powders.
I've been shooting Contender pistols and adding to my addiction for almost 25 years. I have never bought anything new. I have only bought two complete guns, my first and another in a gun shop at a steal of a price (I mainly bought it for the barrel that was on it). My other three Contender frames and 14 or so barrels (not to mention scopes, mounts, grips, forends) were all bought used either at gun shows, on forums like this one or from local individuals and gun shops. If you go with an Encore though, you might have to be a little more patient for what you want to show up on the used market simply because they haven't been on the market as long. The same applies to the G2 frames and grips, although the barrels interchange between early Contender and G2 Contender and are plentiful new, used and custom.
If you go the Contender route and think you might eventually hand load, you might go with 30-30 AI (Ackley Improved). It is a great caliber for the Contender and you would be able to shoot factory 30-30 loads in it until you get set up for hand loading and also if you get into an ammo situation in the field somehow. You would also be fire forming brass for when you eventually hand load 30-30 AI.
I hesitate to go into what to watch for when buying used but you did ask.

Don't let this make your head spin because none of the differences I'm about to lay out are a show stopper. On the classic Contender platform, T/C went through several iterations, mainly concerning the hammer configuration and the trigger/trigger guard. There is an early hammer which has a cheesy hammer block safety that nobody ever uses because if you forget and drop the hammer on it then it is damaged. This same hammer also requires a screwdriver to switch from the centerfire firing pin selection to the rimfire firing pin selection (Encores only have a center fire firing pin if you were curious). Later hammer configurations have a selector lever on the top of the hammer with three positions; rimfire, centerfire and safe. I have both and both work just fine although the latter is a better design. Another difference you will find among classic Contender frames is the "pre-easy open" frames and the "easy open" frames. The differences here are the shape of the trigger (no difference in function) and the location of the trigger guard hinge pin. Relocating the hinge pin gives you a bit more leverage on the trigger guard spur when breaking the action. I own both and to be honest I cannot tell much difference between the two, but I have a pretty strong grip. They both take more pull than children or small ladies are likely to be comfortable with (to put it in perspective). I notice more difference in effort to break the action when I use early barrels with a single piece locking lug versus the split locking lug that is on most barrels. I will not go into the specifics of barrel lockup because you are unlikely to run into many solid lug barrels and I believe that ALL 14" barrels had split locking lugs.
One thing that must be mentioned concerning the classic Contenders vs. the G2 in a hunting situation. If you decide not to take the shot and then lower the hammer, on the classic Contenders you must depress the trigger guard fully to reset the trigger. This has on occasion spooked alert game for some folks. The G2 and Encore do not have this weakness, but it is said that their triggers take some work to be as good as the classic Contender's trigger out of the box. I do not own a G2 so cannot attest to that first hand. There are definitely G2 fans here and one of them would be more qualified to explain its virtues.
That's all I will lay on you for now. Don't be scared off by these subtle differences, as I said none of them is a show stopper.
Curtis