A freind of mine has an '82 or '84 I think, KDX 200 (air cooled)... I have an '01 KDX 200... I can tell you there are massive differnces in our bikes design and charachteristics (mine is faster, taller, more responsive, less finacky, and more stable.... oh, and my brakes actually STOP the bike)... I have never ridden an older liquid cooled though...
obviously, they didn't change the design to make it suckier.. anything '95 and newer will be the same design and a well cared for '95 will be a better deal than a beat on '02, even if the price is the same...
good luck, it's a great bike either way