Russ, best thing to do is have a look around the paddock at the races and keep tabs on what people are riding. Try to cadge a go on as many as you can, then decide.
Complete bikes are OK, but can sometimes be a false economy cos you'll end up changing most of the bits.
If you want a pure tin can race bike, Crupi, Supercross, Avent, Free Agent, Haro, Redline, SE, etc are popular.
If you want something with some cro-mo soul/roots, then the usual suspects.