I would suggest you pick up an 03-05-ish track bike, not a salvaged street bike. The reason being is that by the time you have put some goodies on the bike, you will quickly surpass the minimal cost of an already prepped track bike. I just sold two '03 R6's with full suspension setups (Ohlins/Penske/RaceTec), clip-on, brand new Vortex rearsets, PCIII's, Quickshifters, radiator guards, race body work, one had a fresh paint, gear indicator, one with decent slicks, 1/4 turn gas cap, etc.)... they sold between $2K and $2,500. Already safety wired, motors still ran strong. And with that minimal cost for a bike, it is PLENTY good enough to throw down expert lap times
You can find them on the various race organization's forums around the country. Check out WERA for some nice ones. But some of the other clubs have smoking deals too. See what racer's forums there are for the closest track to you.