a 10 second car for 6k better check to see if it even comes with a motor and tranny! By the sounds of things, and I know from experience, if the car is a true dedicated racecar then it will have atl least that kind of money in the car alone. A role cage is street legal, as are most other things found in a racecar.
The point of modifiying a car to be raced is only to improve safety and the likelyhood of survival in the case of a crash. The car must have lights (headlights, taillights, signals etc...) tires that don't protrude from the body, ie tacky tipical firebird jacked up and tires that stick out 4-5 inches, seet belts are a must. If it's legal to go 10's in the 1/4 then it's probably legal for the road. Fiberglass parts are ok too, I think.
More details would be great, but it sure sounds like a blast. If you're good for the 6k make sure the motor isn't screwed and that everything works like you do for most cars. Just a helpful hint, most racers don't put a lot of time into the body soo be on the lookout for bondo! Major rust probably wonn't be a problem but check anyways. Be very patien and thourough. If it checks out always offer less that they waht!!
Good luck.