There is a reason a person stops on a project. Usually once they get into the car and see what all it needs they bail out, lol.
With the visible rust there has to be really bad rust issues in the usual places, cowl, floors, trunk, tail light panel, quarters, etc.
Without it being somewhat of a special model it would cost way more to rebuild than find one in better shape. Yes you can fix about anything but takes lots of funds or lots of your time and funds.
Like stated you need more info, door sticker, Marti and lots more pics.
BTW you should never leave a Mustang sitting on stands or a lift with the wheels just hanging. It pinches the rubber bushings in the upper shock mount and they split it also can split the bushings in the radius rods. Ford manuals give correct way to lift the car. Probably has leaking tires so he put in air to keep them from cracking when flat.