If you are just beginning, then 17.5 buggy or 17.5 short course are probably the best classes to get started in, purely from a driver development perspective. I've been racing for a few years now and am still running 17.5 buggy. It's hella fun.

Mod Stadium Truck is also a lot of fun and has been doing well lately, as Tony mentioned. A 10.5 motor is something you could grow into. Just turn the throttle endpoint down until you feel comfortable with it, and as you improve, you can really start pushing the truck.