I have a ratty little Yamaha Razz 50. It's a piece of shit, but I'm in it for less than $400, including license plate and title transfer. I have a spare Razz that I pulled out of a dumpster, and it runs too, but with no title I just keep it for parts. On the plus side, I don't care if it gets knocked over!
For sure you want to get something street legal, and get a plate for it. In the not-to-distant future, off-road pit vehicles will be banned entirely. Already you can't leave the fenced-in pit area at Mid Ohio on one, ( I had my Razz impounded there this summer for no licence plate) and Blackhawk will make you buy a sticker if your machine is not street-legal and plated. Other tracks will surely follow with similar rules.
Too many people have caused damage and injury on their pit bikes, so the tracks (and their insurance companies) are just about out of toleration. You wouldn't want to buy something now, then not be able to use it later.
A Zuma or that Suzuki GSXR scooter would be ideal if you're willing to spend the money.