Yep, rust bubbles already, shouldn't look like that!!!; if it was originally a vinyl roof car then there's clear evidence of typical problems already. And if the roof is rusty, what's the rest of the body like where these cars normally suffer from rust? A shiny new paint job can hide plenty of heartache.
I'd take the collective advice from everyone on this forum & drop this car like a hot potato, forget it. You'll be glad you did later. Let the vendor find some other mug to buy it.
As many here have pointed out there are plenty of cars that already have rego etc, so bide your time, attend shows, join the Mustang club & keep your money deep in your pocket until the right car comes up. Be prepared to pay a little more for a far better car with the right credentials from a bona fide seller. It could be another club member, or an importer with established cred that other club members & Mustang owners can recommend personally.