For $500.00,I dont think you could lose. Even if you found something better down the road or just parted it out, you could get your money back.
Last year I bought a 1989 KX500 for $750 off craigslist. Its a great bike and I ride it all the time. But the problem is, I keep throwing money into it. So much, that I would have been better off spending $2000+ for a better bike, that needed less work.
Just my two cents
Daniel in Vacaville, CA
PS. Taking my KX to Arygll Park tomorrow afternoon, very fun