I second the concern re: heavily applied undercoating. I am fine with undercoating, but it does make it harder to quickly or easily ascertain the true condition of the underlying metal.
That said, other than a question mark re: undercarriage condition (which does not have any indication of heavy rusting, but it is still best to further investigate), it is a very nice looking Mustang. If you plan to beef up the engine you may want to do the same for the tranny. It is a c4, which is nice tranny, but when they get a bit too much poweer sent through them a few too many times they tend to not continue to work well.
Depending on where it will be after the sale the new owner may want to add A/C. We added Classic A/C to both our 73 Mustangs and savee very pleased with how well they work.