What stood out to me in that first ad:
one floor panel and rear fenders need bondo,
Floor pans never get bondo. Bondo is filler for small dents and dings, or, in a pinch, it can be used as a very cheap (and crappy) way of rebuilding rust holes in
non-structural areas.
The floor pan on a unibody car (and the Mustang has always been unibody) is a
structural component. It needs metal. If there's a hole in it, you need to cut the hole out and weld in new metal. Using bondo in a car's floor pan is a very good way to end up with a car that folds in half one day while you're driving down the road.
The fact that the previous owner thinks bondo is an acceptable repair material for the floor pan of a unibody vehicle makes me think the poor car is just full of similarly crap-tastic repairs.
Wouldn't touch it with a 10-foot pole.
As for what to get as an amateur, my recommendation is to find something with a clean body and interior but a blown engine/transmission. Bodywork and interior work are more difficult to deal with without specialized tools and knowledge.
ZV