The best first track bike is the one you don't mind crashing. That way you work on riding, not sweating the scratches that may come in a crash! Something like the 2 strokes that come with race plastic is good since they handle crashes much better than factory street plastics.
IMHO I'd get a 2-5 year old 600 that's running well and already track prepped (ex-racer or track day bike). Should run 2-3 grand and need almost zero maintanence compared to a 2 stroke. The 2 strokes would be great too if you don't mind working on them a bit and like Hammer said, get some extra jet needles, I hear they're a little picky with weather and altitude changes.