The 250s require more work to setup, but are faster than a 200 once done.
You need a 88 KX head gasket , .26" thick
remove the silencer
V force II reeds
Carb - #6 slide, 1367 needle, and 38/155-158 jetting.
Once you do that, the motor rips. Especially with a port job. Like any used bike, watch the bearings and linkage for slop.
The bike is heavier, but you can remove about 10 pounds by swapping out to a 90-91 KX 250 rear fender and ditching the metal supports there. The forks are sensative since they only have compression adjustment. You can swap to a 91-94 KX fork easily if you get it complete. Even use your stock clamps. That helps a ton right there!
They have good points, large fuel tank, WR gears 5sp, and a 70w lighting coil. I have seen most ones in good shape go for around $2k lately, but like said watch what you get. When I got mine it cost about $700 to get the rear bearings, axles, pivot bolts and new unitrak link since the entire rear end was gone.