So with 5K you were talking "something like an evo or, S2000, or miata"?

Wouldn't your Mustang be a 3.8L V6 BTW?

Working with a 5.slow would honestly make a really fast project car, they respond really well to bolt-ons. Otherwise if you plan on learning alot, DSM's (Eclipses/Talons) can be made very fast and bought for not much money.