Scca rules:
https://www.scca.com/pages/solo-cars-and-rules
Looks like they have the r in ds, stu, and asp classing. Ds is where the wrx plays I think. Ds you can't do much but you can do a front or rear sway and any tire you want on a stock width wheel. Stu is pretty much all suspension goodies, but no tune (unless they changed to match sth). Asp is all goodies, tune, slicks, etc.
You can see how certain mods can bump you into a class where you may not be competitive without throwing more parts and money at it.
Read through the rules, talk to your friend, decide how competitive and involved you want to be, and have fun
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EDIT: forgot to respond to the spring question. I had MSS and had issues. A lot of other people have had similar issues and the springs don't really perform as expected. I really wouldn't bother with springs until you can do replacement struts too (if you get to that). If you're dead set on them, I'd maybe try the APR or ED. I ditched DCC + 034 for PSS10 and they're definitely a stiff monotube coil, but they ride well; chassis flex is now apparent.
No you don't need anything extra to set toe. I run 0 toe front and .06 total toe in rear for a very tail happy rear.