reknelb,
When buying a used bike my two favorite tests are, a compression test (since it is easy) and I like to take the pipe off and check the piston and cylinder wall with a flashlight for scores, seize marks and general condition. It's easy and quick to take off a pipe and with a 500 you can see alot (huge exhaust port) and can fit your finger in there to see how much the piston wiggles. Also on the KX look through the exhaust at the transfer ports to see if the plating is flaking off. Engine and wheel bearings are cheap. Look for other things that cost big $$. Also, as I recently found out, determine which owner is the most knowledgable bike mechanic. I just got a mint 03 xr100 for my son (from a non-mechanic) and got a few big $$ surprises I didn't expect caused by complete lack of bike savy. My '92 KX is setup for HS and easily bests all the KTM's out there. Still has the original top end (not for long) so they last pretty good. I think the KX is better for hare scrambles but it's hard to argue with a '97CR for the same price. Either one will be a blast. I used to roadrace a fully preped RD350LC. Hope this helps. Cam.