SCCA Solo 2 rules are available on the SCCA web site and can be downloaded for free as a PDF file. There are many classes to run in. Stock, Street Prepared, Prepared, Modified, Fomula SAE. and multiple street tuner classes. Pro Solo rules are the same, but the events are run a little differently than a regular Solo2. SCCA rules are SCCA rules everywhere.
The street tuner classes are partially an answer to the fast and furious crowd, partially to the folks that wanted to "enhance" there cars and have a place to "class" their cars for solo, and certainly have a class structure to have hopped up cars that are "street" cars (NO R rated tires) and not trailer queens.
Street Prepared can be a ball buster!! I currently race a Triumph TR8 in B/SP and have NO shot against well sorted BMWs, S2000's, and correctly prepared Corvettes. Major motivation for the Redline purchase as I will be running A/Stock (probable class assingment by SCCA). If a car is fully set up to SP rules it simply NOT streetable and will arrive and leave on a trailer.
Stock class rules are quite restrictive.
ANY DOT rated tires, cat back systems, shock/strut replacements, and air filter element (only) are allowed. The end. You can't even change the shifter, steering wheel, seat or just about anything else. Frankly I have never agreed with the cat back and R-Rated tires being allowed in stock classes, but my name is not spelled SCCA!!
One can even run stock class and sub classify your car (street tire) to allow for NOT having r-rated tires.
Look at the SCCA site and you'll find the schedule for national events. Google "autocross clubs" or something like "(your town) autocross" and you'll find local clubs. Find one, show up on that Sunday, and take a look. You might want to try it. Except for tires, autocross is really not hard on a car at all. Fun stuff.