Well if you are looking for the best bang-for-your-buck from a beginners track bike, go with a Suzuki SV650 that's already been prepped. They can be had commonly for $3500 or less with some spares (sprockets, wheels, tires, etc).
I'm not saying it's not worth it, you can learn alot on the track with just a stock Ninja 250 with some decent tires. While learning, your main worry doesn't need to be the bike, it needs to be your riding.
You could probably get that 250 setup for the track for a little less money than a prepped SV650, it's just a time/effort vs money argument. You will have to hunt down some used upgrade parts, take the time/effort installing them and tuning them, or just pay a little more and get a turn-key (for the most part) solution.
It's up to you, but either way, best of luck