[84LX89GT]

Probe GT's are another fun car, especially the ones with the V6, those things i think go 15's stock (mid to high 15's i believe is the potential they have stock). ALso the earlier turbo probes cook pretty good, but in terms of replacing any parts it gets expensive because they're basically a mazda MX6 (i think it's built on that platform) and alot of parts are "obsolete" and aren't made in the aftermarket.
I can tell you already that getting a 4 cylinder mustang to go 15's without a turbo is VERY expensive because you have to more than double the power output. the '87 to '90 2.3L's had 88 horsepower and the '91 to '93 had 105 with the redesigned cylinder head, coil packs and dual spark plugs per cylinder (like the rangers had for a couple years before the mustang).
You'd be lucky to get sub 20's in a stickshift, i had a friend with an '89 2.3L and i'd smoke him with my '91 but realistically i'd get humiliated by EVERYTHING else that is driven on the road. I bet the only real match for those cars is like a chevy citation or any 4 cylinder Chrysler K-car.
One car that's made from the same platform and as a result alot of performance parts bolt upto is a Thunderbird Turbo Coupe, they come with 4 wheel disc brakes, 5 speed manual, and i believe 170 horsepower but i may be wrong. The suspension is the same as a mustang, mustang seats will bolt right in, strut tower braces, engine, you name it. You could also buy a thunderbird or cougar from '83 to '88 (fox body years) with a 5.0L non HO and hop that up or build an engine for it that you'll eventually put in it. Also insurance would be ALOT less for these vehicles even with 5.0L's in them.
There are alot of cars built off of the "fox" platform that the mustang is built on and as a result there is ALOT of parts interchangability. The sky is the limit as to what you may want in the future.