I'll just jump in here and say that I bought a 1990 GPX250 (red and black) with 25,000 on the chassis, about a year ago, but it was in good (factory) condition after a single engine rebuild after the second owner blew the engine up, and I paid 3700 for it. I've not had a single issue with it (and I've even managed to overtake 6ers -- just ask anyone that went on the 2009 Gingers Creek trip

)
Since owning it, I've put 30,000 more on the clock, the majority of that was commuting (100k each way, each day), and she's been a treat (with the exception of a bad batch of fuel in Sept). I've had to do the usual stuff (albeit, a little more frequently than most), but I would thoroughly recommend any learner start with a GPX250!
I've learnt so much from this bike, they are easy to pull to bits (in fact, I can get everything disassembled and be looking at the crank shaft within 20 minutes now -- and I didn't know ANYTHING about bikes 18 months ago) and they are easy to get parts for (thanks to everyone else finding trees, gutters or shiny-side-down) if you ever need them.
Anyway, more to the point, the link that Smithy put in above is an awesome checklist, I used it when purchasing this bike, and I would print that out and take it along with you to any inspections. The second you get to a negative answer, ask for an explanation, or walk away (mine had a near-bald front when I bought it, but that was easily fixed).
If you have any other questions about GPX250's of this vintage (even though they've been the exact same from '88 to about '04), just PM me.
Cheers, and good luck!