From the full body pictures, I would say that although she definitely has good muscle, she does look to be in need of some fat. If I were you, I would ask the old owner what she was used for/ how much she worked- that can give you a better perspective. If she was worked a lot, it may just be that she has low fat percentage. A good example is a tb on the track- they're without doubt very fit, but they still have ribs showing because they're all muscle with very little fat. If she was fed with a lot of other horses and low in the pecking order, that explains why she has so much rib showing.
My advice is to get the vet out. Get her feet trimmed (they look pretty long), get her teeth done and see if she gets any better. If she doesn't, start getting more specific about what you're looking for.