hey, Im the owner or an 85 kdx......Check the swingarm, linkage, and wheel bearings while the bikes is on a stand. if there is any up and down slop, adjust the price, they run about 40 bucks to replace (both swingarm bearings , or the linkage bearings) also when you run the bike double check that no clanking is coming from the lower engine case. kdx's make some noise, but if it is from low in the engine and excessive be weary. Remeber to ask if he has gone with an over-bore kit at anytime and ask what size piston he has in the motor.
I love kdx's but these are al problems i have had with my bike, i bought it for 400 dollars as a restoration project. The kdx is a great bike but remember to ask the right questions. I tend to believe i got a good deal because of the fact that i unknowinly bought a kdx240. I found this out when i pulled the enginge because of the clanking noise and realized the piston was 83 mm.....much larger than stock.
Good luck, the kdx is a great bike and it sounds like your headed in the right direction as far as a good price for a great bike.
Tim