To be honest, find the cheapest 250 you can find. Wont matter if its a CR250, KX250, RM250, or YZ250 (Honda, Kawasaki, Suzuki, and Yamaha respectively)
Price should determine this purchase. And dont get the prettiest one you can get, cause chances are you WILL be laying it down.
And factor into other gear. You DONT want to ride a dirtbike without riding pants and boots at least.
Make your first dirtbike a cheap crappy one that you arent afraid to lay down. Learn it, and sell it and upgrade.
Buddy of mine just scored a 99 RM250 for 750 bucks. Its pretty beat up, it runs really good, its perfect to get used to it on, and hes not afraid to rag on it, beat it up, etc.
This is my advice at least. This is what advice was given to my buddy from my girlfriends uncle who motorcrosses and has been riding for a LONG time.