Would a 125 be a good fit for me?

Hey all I'm looking into getting a bike and have been looking at everything from 450s to 125s and everything in between and what to know if u guys think something like a YZ125 or RM125 is big enough for me. I'm a lean/slim guy (6 foot, 150lbs) and power isnt a big deal, i can handle it. I just want something thats light, very fast, and cheap (college student's budget). I guess I just want some assurance in knowing a 125 would be a good bike for me that i can keep for quite a few years.i ride out in the desert, anza borrego/ocotillio wells area