if you are serious about SCCA events, then you need to have a good read of the rulebook. I'm not certain, but your STS and CAI might bump you upto prepared.
other than that. all you really need is tires. some 14 or 15" steel wheels with some beefy winter tires and you're good to go.
i recently bought a beater escort GT to use for rally-x. it has no interior so that would have put me in SCCA 2wd mod class, but locally there are only 2 classes. 2wd, 4wd.
May 2014 ROTM, June 2015 ROTM
for auto-x: '04 Liquid Grey ZX3
for street: '02 Infrared SVT