And I would also suggest going to a few autocrosses as a spectator and seeing in person the kinds of cars people are running and how they do.
If you see something you like that looks like it might be in your price range, talk to the owner about the car and what kinds of mods if any they might be running.
Its a little more intuitive than just going through the rule book, but sometimes its easier to tell what you want by seeing it instead of just reading about it.