Sometimes horses can dislocate the pastern bone below the fetlock.It's called a subluxation, and it may never heal back quite right, and could pop out of place again. Or, it very well could be a simple scar that has really thickened or the ringbone you metioned.
The thing about ringbone, is it can be high or low, or both, and usually is from years of imbalance, whether from compensating for another pain, or from crappy hoof care, or lack of any hoof care. It's also most likely to occur in the front feet, as all hoof pathologies seem to start there. However, it's not impossible for the hinds or even only one side to suffer, but usually, the hinds aren't the first ones to get ringbone.
My thoughts go to perhaps the old calcified fracture, something like a bone spavin in the hock, or the subluxation, which would do something similar, and try to fixate the joint by producing more bone, and creating a visible deformity above the hoof wall.Again, it's not very common, but if she stepped in a gopher hole, or stepped off a trailer wrong, it could have happened. If she's stilll limping, I'd scrape up the money for some x-rays before I committed to owning her. Her usability may be severely limited, even if it is "just" ringbone.
Ringbone is often "osteo arthritis" and is a permanant, though sometimes manageable, condition that tends to worsen. It's arthritis, and requires special care, and she's not going to hold up to heavey work if that is what you had planned. Also, 5 is a bit young to have arthritis, especially if she's that wild, I doubt she's had much of a riding career, so I'm back to thinking it's an old injury.
My last thought would be an old joint puncture and the bones are rubbing on each other after an infection eroded all the soft tissues between them, and agian, her future is going to be limited, most likely.
Really, have the vet to a thorough work up is the best advice I think you could get around here.