1) You shouldn't buy a NSX for cheaper than 30.000 $ because finding a good condition car will be difficult
2) The 1991 model NSX's has something called "snap ring" (see FAQ
http://www.nsxprime.com/FAQ/faq.htm for more information or search it) - this you want to avoid because if the tranny fails it does cost a little. Don't trust a 1991 NSX seller if he sais that the car is in snap ring range but it's fixed, you should require papers for the job done to fix it etc. Later NSX no snap ring problem
3) There are mainly three types of big changes on the NSX through the years, the first ones are called NA1 (from 1991 - 1997) - the original. This car has 5 speed, 3.0 l. V6 engine etc. Then from 1997 - the last NSX, the NSX was modified and has the name NA2. Many people claim these NSXs are much better, with bigger engine 3.2 l. (still a V6) and a closer ratio 6-speed with bigger OEM wheels, brakes etc. And the 3rd and last big change was in 2002, when the exterior got facelifted, but still 6-speed, V6 3.2 l. engine and so on.
4) NSX is available in both targa roof version and coupe version. If you want to track the car, a coupe might be a better choice, whatever suits your needs. I know coupe is extremely rare in the facelifted 2002+ version, almost every NSX sold from 2002+ is NSX-T.
These are simple details about the car. I would (and will) buy the latest model I can afford.
Maintance; On NA1 NSX's, the clutch is cheaper to replace than NA2 NSX's. (I don't know the exact amount for the clutch parts/labour because I haven't changed a clutch yet). Timing belt should be changed according to the instruction book/ service book (whatever you call it over there lol) guidelines, a failure in old timing belt can damage engine very hard. The rear tire wear is faster than family cars lol
You should read the FAQ, if you can't find it there, you may ask about it here
Do you track the car? Are you planning to use forced induction? Tell us what your needs are