What dirt bike should i get.?

I am 15, 5 feet 6 inches and i weigh 135. I am going to race. if i were to get a two stroke i would get the kx100, kx125,or cr125. If i were get a 4 stroke it would a kx250f or crf250r. Only those bikes.I already know that i am getting a 250 later on down the road the problem is if i can handle it right now. So i should get the two stroke now cause i can probably better handle it, and then sell it later and get the 250.Or should i get the 250 right away. And if i were to get the two stroke first should i get the 100 or 125. ------