You've got a few choices as far as SCCA classes go:
FS - "stock" - allows catback, drop-in air filter, replace shocks, swaybar, lighter weight wheels (must be same size as available on car), and R-compound tires
ESP - "street prepared" - any exhaust mods, may replace intake, underdrive pulleys, tuner, can change to an aftermarket clutch, can replace seats, any springs (may switch to adjustable coilovers too), camber plates, strut bars, radio + AC delete allowed, any brakes available on that model car, can change shocks and swaybars, may replace panhard bar with a better one or change to different type, any size wheel, R-compound tires, can add rear spoiler / wing.
SM - "street mod" - wouldn't call it unlimited, but tons that can be done. internal engine mods, can supercharge or turbo, coilovers, watts links, etc are all fair game. MUST retain interior. this is a big $$$ class. R-compound tires.
STU - "street tire "unlimited"" - more or less same rules as ESP, but with Street tires. I believe also has a tire width rule (check the rule book)
CP - full out racecar. gutted interior, nearly unlimited engines (weight of car based on cubes), able to move suspension points + add 3link / torque arm / panhard / watts link. R-compound or Bias Ply slicks. This is where the real fun is!!!!!
LOOPHOLE. If you have a California Special equipped car you can legally put on the FRPP handling package and power pack to make a "shelby clone" and run in FS. The Shelby GT is THE car to have in FS right now and should run with the track pack cars. If you can do that, you can go extremely fast and have a lot of fun. This is by far the cheapest of the classes to run.
Mustangs are not very competitive in STU. One of my buddies has an 06 GT and he did "ok" at the Houston national tour against a tour STU crowd, but certainly isn't "the car" to have. He'll be running ESP at nationals...
ESP - Mustang has a ton of potential to play here. BUT keep in mind that a truly competitive ESP car will not be a streetable machine. If you don't want to play (or win) on a national level, you can have a heck of a lot of fun here.
SM - Mustangs aren't competitive until there are rule changes or a breakout of pony cars from the IRS field. But, this is the only place you can play if you supercharge your car...
CP - going off the deepend here on $$ and lose any semblance of streetability, but it's about the most fun you can have with your pants on in a car. Racing nationally one of the best and most supportive group of racers you'll find.
Rule book is available for free to view here:
http://cms.scca.com/documents/Solo_Rules/2009_Solo_Rules.pdf
Anyways, sorry for the long post!! Been doing this for 9 years now, the last 5 in CP (doing reasonably well nationally I might add

). Welcome to the sport!