Am I reading this right? Even if your car comes from the factory with a rear sway bar you have to remove it to run in stock class? That makes no sense at all.If you want to stay stock, then you must use your stock springs and you can't use a rear sway bar.
My advice: Do an event to make sure you like it. If you're like everyone else, yoll be hooked immediately. Get some new shocks, the AGXs are good. Leave everything else the same. Use the street tires you have. Trust me, you'll be a better autocrosser in the future if you learn on less forgiving crappy street tires. Use the rest of the money for that Evolution phase 1 school. It's worth every dime.