I'm looking into buying an old beat up car?
"Soup Up" is a rich man's game. If you just want to learn, then I think this is a great plan, but I would look for a car that would be something worth having without you supplying $10,000 to "soup it up" as if somehow that was going to improve your life.
It's hard picking something from the 90's. Almost all cars sold in the US were front drivers, and they're not much fun, and you've already got a Mustang. You might look for a miata, or 240sx. The front driving Eclipse/Talon/Laser with the turbo would be fun.