Some quick things to check:
front and rear shock condition ( if it feels bouncy like a pogo stick it will need rebuilding or replaced)
bearings in carrier and front axle. Ask to jack the trike up and turn wheels by hand, listen for noise, wiggle axle around to see if there is any play. Also free spin the axle and check if the axle is bent, or rims are bent.
Also while trike is off the ground check swing arm bushings/bearings. Attempt to wiggle swing arm and check for slop or play. Same with front forks, check the neck bearings.
Condition of tires.
Look frame over for cracks or repairs. A 250r frame is robust but it never hurts to look things over.
Under and around engine for oil leaks.
Test the brakes, if possible look at pads.
Chain and sprockets. If o-ring chain look for missing o-rings, stuck links. Sprockets look for worn out teeth on gears, will have a hook shape if severely worn out, sharp tips.
Look inside gas tank for rust.
Check gear case for oil. Pretty sure the 83-84 250r had a little screw you remove to check if there is a slight amount of oil in the case. Someone please correct me if I'm wrong.
Plastics, look for cracks, repairs.
If it's running make sure to take it for a test drive if they allow. Cycle through all the gears, open it up and verify engine is running properly, listen for misfires, excessive vibration. If the engine is warm or hot when you get there, more than likely they are trying to hide worn rings or low compression. When a motor is warmed up or hot, the rings will have expanded from heat and the usual tell tale symptoms will not be apparent.
This is a just a quick list of things I thought of. I'm not an expert with two strokes so hopefully someone will chime in with things to look out for. If you start noticing many things wrong, don't be afraid to point them out and offer a lower price since they will need repairs.
85 Kawasaki KLT 160
85 Kawasaki KLT 110
82 ATC 70
2000 Scrambler 500