Buying a used bike

Thanks to anyone taking the time to read this.

I am wanting to get a dirt bike, and this will be my first bike ever. I dont know if i should get a 125 2 stroke, or a 250.

If i get a 250 im debating on the 2 or 4 stroke but really want a 2 stroke.

is a 250 2 stroke to much? i weigh about 120 pounds soaking wet lmfao, and will be 18 in september. 

The next question is whats a good price for used bikes, or whats a good used bike. im thinking a yz125 or yz 250 any year after 05.