So far all of the bikes posted in this thread are overpriced, including the mid 80's Kawi for $500. A word of warning on any 20+ yr old MX bike: should you have a motor issue, as a bike of that vintage would be prone to do, you will spend nearly as much to fix the motor as the entire bike cost you.
You should be able to pick up a mid-late 90's 250 for around a grand. Or if you want a bit more bike, there are plenty of 2000-2002 bikes not selling in the $1500-2000 range. The improvements in chassis and suspension designs on bikes made in the mid 90's upwards will be well worth waiting and saving up some cash. Another benefit is having a bike less long in the tooth and less likely to have gasket, bearing and seal issues among other things.
For reference, I posted my cherry 02' CR250 for sale a few months ago for 2k and didnt get any interest at all. My bike has less than 110 hours on it, a fresh PC motor, tires, chain, sprockets, plastics, 3 pipes, 2 silencers, 2 filters, ect.....
Here are a few bikes that stand out to me as having been great bikes both in their year of manufacture, as well as today:
Kawi: 1996-1999 Emig Era bikes were notoriously strong off the bottom and had decent suspension for the day.
Honda: 1992-1996 McGrath Era bikes had some of the gnarliest 2 stroke 250 motors of all time. Great chassis, the last of Hondas steel frame. Suspension was on par with the rest. Stay away from 1997-1999 Honda 250's. The worst production MX chassis in the last 20 years.2000-2001 had a much better chassis than 97-99 with similar geometry to the 92. 02-07 one of the best chassis Honda ever made. Motor is very electric and linear, not the punchy powerhouse of hondas of old. Still not sure which I like better. A motor that rips your arms out and you cant ride more than a few laps on or one that builds power controllably and predictably.
Yamaha: Any bike 90’s up will be solid. The 94s had amazing power and had lots of aftermarket support like Noleen cylinders that bumped displacement over 50cc. 95s were the steel framed basis for what Yamaha is still selling today. Great motors, KYB suspension worked really well. 00-04 great all around bikes. Reliable, easy to ride, very workman like. 05-present aluminum perimeter frame, very nice KYB sss susp.
Suzuki: I would stay away from anything before 1999. The 99-00 were a lot better than previous models but had fidgety chassis and weak motors. 01-08 were the best 2 strokes Suzuki ever made. They had motors that were easy to ride, fast and reliable. Showa suspension than settled down their reputation to have headshake. Much better manufacturing as well.
KTM: The newer, the better. Dont bother with anything more than 10 years old.
Try checking classifieds on thumpertalk, ktmtalk, ect.
Hope this helps.