Welcome to MW!
It's amazing how many people have modded these cars that it may be a challenge to find a relatively low mile, good condition and unmolested Mustang of those years - but they are out there. If it were me, I would try to find one that wasn't modded at all or at least not very much.
Once you find a car you like, come back here with the details and we can help you determine if it's a good buy or not.
Also, what are you planning to do with the car? Just tool around in it on the weekends? Mainly a drag car? Daily driver?
Thanks and good luck,
E