as far as building the car:
Figure out the who you're going to spend most of the time running with and get their rule book. That will outline what you can and can't do to your car. Unless you want to just get bumped to an unlimited class.
Once you have the rulebook my rule of thumb is:
1) chassis, make it as stiff and safe as the rules say
2) determine tires, budget/rules
3) suspension, rules/tires

Everything else comes after that. The most important part is the nut behind the wheel.