Have you have any experience with a bike before? MSF class or dirt bike experience?
If not, (what I would recommend) start on a used ninja250 and tear it up. Afterwards, sell it and get a 600cc bike. Cause if you buy the 600cc one now, and about the time you tear up the 600cc (in comparison if you start on a 250cc), you’re gonna want a bigger bike. This way, you develop all your skills then you can have a little fun.