This is a tough question because if you were to buy a smaller motorcycle such as a Honda 250cc engine. Then you master this bike, you will want to move up to a quicker and larger bike in just a matter of time. So if you can find a used 1st bike and then move on to a bigger bike when you are ready, this would seem like the logical way to go. Do you know of someone that will loan you a smaller bike to learn on?
Good luck