I've ridden my father in-laws YZ250F and I tell you what, it is a monster. I was thinking, 50 more cc's isn't much...yeah right! Its a great bike, rides stiff as a board though. I'm sure it smooths out at higher speeds.
From what I gathered, these are race bikes. The top ends need to be rebuilt at specific intervals. These are just my opinions and assumptions though. Hopefully someone that has one can chime in.
You'll end up putting at least a couple hundred on up in parts on it to get it street legal.