Originally Posted by jambo

Of the 2 original choices I'd be thinking SV650.
The main reason as already discussed is availability of spares.

The others aren't wrong though, any Super sport 600 from the late 90's onwards in good mechanical condition makes a great track bike. The main questions should be:
1) How much money do I want to spend on a bike that is realistically almost certain to be crashed?
2) Do I want to race, in which case are there any classes that I'd like to race in?

With point 2 you could find that some of the pre-injection or steel framed classes are cheaper to get into because you won't be next to a 2013 spec bike with full Ohlins on your 2001 GSXR.

Jambo