I love the '80's KDX's as you can see by my stable. I just got back from riding in Colorado and the '80-175 and the '85-200 did great. I bought the '85 last August as a non-running basketcase. Parts are pretty easy to come by on eBay and Fredette Racing Products. The '83-85 are similar with mikuni carbs and no disc brakes. The '86-'88 have Keihin carbs, front disc, and KIPS exhaust valve. Then in '89 they went to water cooled. If I had my choice of any year it would probably be the '88. The brakes are great, bigger front forks, but also a higher seat. You can't go wrong on any year as far as reliablity. Every used KDX I have seen needed swingarm bearings. They can be had from Pivot Works, but you may have to call them for the older models. They didn't have an off the shelf package for my '85 or '88, but they found the right pieces for me, no extra charge. To check the swingarm, **** the bike on the kickstand so the rear wheel is off the ground. Grab the rear wheel and see if the is any play, side to side (swingarm bearing) or up and down (linkage bearing).
Good luck!
Nice job DRN...The **** is becasue I didn't say to lean the bike on the kickstand, I said to co_k the bike. What was I thinking? I've got to get my mind out of the gutter.