Hi Rob,
Electric onroad is split into three main classes, which use the same chassis and bodies but different turn motors. 21.5t is the entry point, with open modified being the fastest (4.5t/5.0t motors). There are also two other classes (VTA and GT) which use the same chassis, but different bodies and motors as well. GT uses a locked timing 17.5t and sportscar bodies, whilst VTA is muscle car bodies.. but can't for the life of me remember what motor it is! I think 21.5t.

For your situation, I would look to a 21.5t motor, and any 190mm body. Don't worry too much about having the most whiz bang motor, even in 21.5t practise is key to quick lappery. The club will be more than happy to accommodate you coming along and having a crack, gotta start somewhere!