Welcome mate! Good choice - a 600 too start is ideal. A lot of guys make the mistake of trying to track/race a litre-bike first up, don't learn racecraft and bike setup etc etc, and get disillusioned.

If you really know what you're doing (and you need a fair bit of mechanical knowledge), you can build a bike from the auctions. It's fairly risky, as you don't know for sure what the damage will be. It will also be LOTS more expensive than buying one already done. And I mean HEAPS more exxy! If you're ok with that, it can be a very enjoyable process. And you know the engine will (normally) be low kms and not thrashed to within an inch of its life on a racetrack for years.

The flip side is buying a track or racebike gets you a lot more bike for your money. You will know it's good (except engine wear), and it'll usually have lots of goodies on it already that cost heaps to buy new (full exhaust, Ohlins suspension, steering damper etc etc), and will be lockwired and have engine covers and shark fin etc. Perfect if you don't want to tinker.

I would steer clear of the 675 unless you specifically want to do BEARs. They are fairly unreliable, and parts are expensive and hard to get. The R6 and ZX6 are the racebike of choice. The CBR600 is a better trackbike for newcomers to learn their craft on, so don't count them out either. Having owned two gixxer 600 racebikes, I cannot recommend them for anything.....