Yup - see that's valuable info. The twins (FRS/BRZ) are probably the best and most popular car right now for CS (Stock) and/or STX (coilovers, etc). STX prep won't be cheap though.
The stock Prius tires suck for autocross so you'll definitely want to ditch them quick. You want Bridgestone RE71R in either class. I believe the fast cars are running 225/45/17 in CS and 245/40/17 in STX (allowed wider wheels). There's a ton of info on these cars on the interwebs - I know several people that race them if you want some tips on what works and what doesn't.
I would suggest adding a front sway bar and Bridgestones to an otherwise stock car. That should easily be competitive locally in the right hands ..... so it will only be the rookie driver holding it back