I disagree. The Suzuki/Kawasaki joint effort in the 250f model was troubled, to say the least. Quality assurance seemed to be an issue with them. Some were pretty good, others were notorious for dropping a valve. THE most reliable one has been the '03- YZ250f (for the lites class size) and of course the YZ450 for the big bike class. However, it seems the 450's from Kawasuzki weren't plagued with the same problems as their little brothers. There have been a fair share of the '02-04 Honda CRF450's with valve problems, as well, and their CRF250 was a pretty short life motor wrt the head.
All in all, a "cheap" 4 stroke mx'er is a tough find. If it's abused, or ridden hard, you will most likely pay more in the end after a topend rebuild (valves, cams, chain, piston, rings, etc etc add up to thousands $)....
I'm with PJ, the CR (or even YZ) 250 2 stroke would be my choice for a "cheap" used dirt bike. Complete motor rebuild less than a grand!! I'll take that, with light weight and power to boot. Sure, it's not a flashy 450 4 stroke, but the fun factor is through the roof.
My $0.02 (or 3)