2003 mustang gt supercharged 91 or 94?
 

What should I use when I buy the vehicle? Is regular gas out of the question?

Depends on what the tune is written for, err on the side of caution if you don't know and go 94 maybe.

I would think with any supercharged vehicle regular gas is pretty much out of the question, if you're buying forced induction and worrying about running regular gas to save a few bucks, its likely the wrong vehicle for you.

If its stock 91 is what Ford recommends for supercharged vehicles. Aftermarket tune could be anything.

What they said, min 91, 94 on the safe side.