You all have some weird classing rules. At Nationals, I'd bet 99% of the people there were on R-rated tires. The STS/STX crowd almost all ran on Azenis. This is national championships.
For autocrossing, if you are running an ST class, use Azenis. If you can run 15" rims, you should be able to get them for under $70 each tire. Just buy a second set of rims. 15 are cheap as hell! I wish I could have run them.
For stock classes, if you can run more than 2deg camber stock, which none of our cars can, use Hoosiers. If you can't run the camber that high, which we can't, use the Kuhmo Victoracers or Ecsta V700s. They rock.
Stock you should be in GS with an MP3 and I think HS with a P5. Modified slightly, such as CAI, Camber plates, High-flow cat, body kits, etc. would put you in STS. Add LSD and you move to STX. Do those mods on race tires and move to FSP.
SCCA Clasing should be really easy. I know some regions run weird classes like a street tire class, and other BS, but don't count on seeing that junk at divisional or national races.
Some local regions tend to cater to whiners that can't seem to deal with the already overdivided classes and add more to confuse things. To each his own, but I'm pretty sure the SCCA has a good reason for the classes they have.
Good luck and let me know if you have any Q's