Honestly the beauty of autocross is you can run whatever you bring and still have a blast. Get your car in good working order to pass tech and then just drive youre best. Youll be surprised at how much improving you need as a driver before youre at the limit of your car.
That said the SVT suspension would be my #1 first upgrade
"Ford Focus ZX3 SVT: The official car of Senior Skip Day and the churning white froth of teen apathy and the crisis that arises between individuality and conformity." - RCR