Most parts to rebuild an early Mustang are available and reasonable in price. It is not for the weak of heart though. Major areas of concern are the inner frame rails, torque boxes and floors(as noted above) and also the panel under the cowl. Yes there are more spots that go bad but those are very common. A Mustang convert in #1 shape would bring $20,000 - $35,000. But to have someone restore one in bad shape to #1 would cost that much or more. Where is the car located?