You're not going to like the 5s unless you're traveling in a straight line. That's way to much for racing. Fun for bashing, I guess.
IMHO I think 5S is too much for that motor in a buggy. You're talking ~47000rpm, which the motor can do, but you'll be way past the efficiency range and there will be a lot of AMP draw getting it up to speed.
“Everyone has a right to be stupid; some people just abuse the privilege.”