If you buy under a 500cc, you are going to be kicking yourself in 3 months.
Just because you have the power doesn't mean you have to use it. That's why there is a throttle.
Buy a 500-650cc bike, and be absolutely careful while you are learning... and afterward too.
I learned on a 250, but that's when i was 12. Any responsible person should be able to handle a 600.