I am mostly looking for 87-89 Carreras but i've toyed with trying to squeeze into a 95 high-miles 993, which you can find between 25-28k,
but it will have issues. For 30-40k you should have no issues getting a decent if not nice 993. Yes people brag on these boards about how they keep their value but hey like any car, the older it is, the more it depreciates. I am pretty sure you wont be able to get a 97 or 98 C2S which are the most desirable models those are much more.
For example you could get a 97 C4S - this one is pretty nice with about 60k and a recent top-end done by a Porsche dealer in Burlingame:
http://www.burlingameeuropean.com/detail.cfm?ID=214124
I saw this car and it is quite nice. Its probably a driver not a show-car
but I am sure would be fun. Couldn't take it for a test drive since the owner needs to be present. If you want me to take it for a drive on your behalf let me know
I know of a very nice red 964 for sale in San Louis Obispo also for 20k... if you want to check that out for me