I think theses classes would be a good starting point:
Stock 17.5 sedan no boost
Gearbox class (st, mt, 2wd buggy,4wd buggy, SC, ect)
When u get more entires, then u can separate the gearbox class into specific categories, then maybe add VTA, GT10, GT8(if large track), 1/12, 1/18, F1.
Basically keep the rules simple as possible so it's easy for people to join in!
Good luck with your race program!