Originally Posted by dvorak
I wouldn't worry about engine size and just go for a bike that is in good condition in your price range.
I don't think it's much harder to go from a 600 to a 1000. The main difference is clutch, brake and throttle control for low speed handling which is more of a concern for street riding. But that's nothing you can't learn within a few hours in an empty car park. The power difference is just a matter of throttle control.
On track, having the power there without the skill level to match imo is a recipe for disaster. A litre bike is nothing to be sneezed at, and if you're not confident or fatigued or don't have the best throttle control in hairy situations they can and will spit you off real quick.
Hone your skills in on something small and go from there