If you're looking for something to zip around on, and ride trails, the KDX is the bike for you. I'm 6'6" or so and 240 lbs with all my gear on. My bike has no problem pulling me wherever I want to go. I don't do any MX stuff at all. Strictly wood riding. My bike is stock except for a fatter rear tire, and jetting.
How much should you pay? Sorry,..dunno.
I'd look for a newer bike though. '95 or newer probably. A 91 KDX can be tough to find parts for.
good luck,
joe