Been autocrossing for 10 years and I have a 97 Cobra (Lightly ESP prepped).
Lets talk through this.
First off, some questions. I like to figure in this way.
Q1: How competitive/goals?
1) Fun at local events
2) Able to lead at local events
3) Competitive regionally
4) Competitive nationally (able to shoot for a national championship)
Q2: How much work?
1) Buy car, align/replace tires and worn parts => Drive
2) Some bolt-ons
3) Hold on to your hats, its time for an overhaul! (swapping between years OEM parts to build hotrod and some aftermarket)
4) I pride myself on my car being a higher percentage aftermarket than factory parts in total parts content. and LOVE fabrication.
Q3: Budget. Please quantify expense expectations and limitations. (Note: consider your goals in Q1 and Q2)
You state "Only 3 seconds off", 3 seconds is an eternity. My point isnt to slam you as a driver, it is to point out that I doubt 3 seconds is all the car. If you have the chance, have a known hot-shoe (national level if possible) drive your car. Ride along with them if at all possible. If you are just starting out, I would wager that you have further to go then you think. Hence, seat time, seat time, seat time. Dont fall into thinking you have plateaued when you havent. After 10 years and being considered a fast driver, I have a friend I can count on to be a half second faster (at least) than me in my own damn car! (that said, he did get a fancy jacket in Lincoln the other year)
That being said. I would stick with the car and get some seat time for now while figuring out what platform you would really like to get into. This is even though the Mustang isnt the best learners platform. Just keep an open mind that it might teach you some habits you have to unlearn. At the very least if you do change cars, dont go mod crazy for a few years.
An SN-95 Mustang isnt a good platform for SCCA these days. IF you are considering 3 or 4 on question 1...
GS: the Focus ST will mutilate you.
FS: (if V8) Newer mustangs and camaros will mutilate you.
STU: As if! Not a chance.
ESP: Got $15,000 to develop a car (and install V8) that may not be relevant shortly due to newer ponycars filtering in with big power? oh yeah, a set of tires is $1300+
SM: Good luck beating the AWD boost buggies.
CP: Gut and totally reengineer, removing any resale value in the car whatsoever. Seriously, start with a salvage gutted car and reinstall as few factory parts as possible.
Those are some of the reasons that my Mustang is in storage right now and I am racing my Miata in STS. If I had it to do again, now that street tires happened to stock, I would do an ES Miata or MR2 Spyder.
If picking a new car, best advice... read some forums for upcoming cars and read the most recent nationals results. Pick a car that you like that at least trophied at nats in a street/stock class and go from there. That is, if you want to be competitive. THIS IS YOUR BEST BANG FOR BUCK OPTION. (especially when you consider that the car retains resale value)
Hope you enjoyed my novel.