There is nothing special to check that I am aware of, just the usual maintenance items. Especially check the swing arm for play in the bushes, wheel bearings and sterring head.
The KDX will run for miles on the same piston and rings but a 98 should have had one new set of rings by now, especially if raced (saying that my 98 is still on the originals but about due for a change). Compression test is the best guide here.
Look at overall condition and how the owner has maintained the bike. Is the airfilter clean, whats the oil look like etc.
Hopefully it has at least got an aftermarket pipe on it and possibly the original too. If you haven't ridden b4 the std pipe is a good intro, but you'll be wanting the extra power soon.
Good luck:)