Decide on your goals for the car and yourself when it comes to autocross. THEN mod the car accordingly.
Because it will suck if you decide to do a modification THEN decide to stick to a class only to realize you can not compete in that class...
I find pleasure in fabricating and improving things in general. You can not run rod ended rear LCA's in ESP, BUT I have left over tubular material to make some...so guess what, I'm making some. Does it take me out of ESP? Yes. Do I care? No because locally there isn't a big ESP class AND I never plan to do any national event. I will do all local stuff (which no one will care if I have rod ended LCA's in ESP) or driving schools etc.
This is a hobby that get e in the garage to blow off steam from work and clear my head. So IMO when I have "rules and regulations" to build to, I will get annoyed. I like the fabrication freedom. Then again, I don't have the wallet to be competitive in CP class either. This is something my wife and I do for fun.
When the car doesn't break, we improve our times throughout the day, we by default have fun together.