if you got the guts, go for a 250 2-stroke, with that kind of budget, your looking at pre-1995, they run bout 1200$, i know your budget it small, but you will need more money, trust me
you will find 125's blow-up too much, and not alot of power specially for your size, 250's are pretty endless when it comes to power, im 6'1, and 185-190 lbs, when i went to a 2000 CR250, it was a steep learning curve, but it paidoff once i learned the bike, and it gives you a good workout for sure
when im done school itll be CR500 time! lol, suicide