fox body mustangs are by far the fastest budget cars possible. 12k will get you a 9 second car. I build mine for 7.5k and it will dip into the 9s on spray. my buddy designs turbo kits so he could design you a system with all of the bends and everything for whatever application you want to run. he has a stock block 302 car making 430/520 at the wheels on 7lbs and it rips up lambos and vettes on the highway. if anything, a mod motor car is the worst thing to start with because the 2v platform is very very hard to really ramp up. my buddy did full suspension, T56, wheels/tires, some weight mods, cams, exhaust, and a T-trip on his 01 and it does very well but he has put more than you want to spend into it and its not an ultra fast car. as far as going F-Body, you could pick up a decent LS1/T56 car for around 6-7 grand and do heads, cam, intake, exhaust, and suspension and you'll have a nasty little street car. once again though, this could blow your budget if you do things the Right way.
old mopar cars in good condition will destroy your budget and you might spend quite a bit of money trying to make an old mustang coupe really fast so maybe a Nova would be a good car to look into. something cheap and abundant will please you. once again though, the fox body will be the fastest by far on a budget. power to weight is gangsta.