Get a good street brake pad. Then get the very inexpensive brake master cylinder brace. Those two things will give you a great autoX brake set up right off the bat. You don't need a big brake kit because your brakes will never get hot in autoX so don't pay the big price.
Consider exhaust if yours if rusting out. Consider the upgraded ECU + boost jets as soon as you can. Once you have the ECU you will wonder why you didn't get it sooner.
It's a short list, but this will give your car a great performance boost for not too much money.