I can't tell much with her conformation & all, but she honestly does not look like she's taken well care of. Her coat is dull and her legs are filthy.. and I know you said to ignore her surroundings, but it's EXTREMELY difficult to. It's not necessarily the mud, but the whole thing screams a bad situation to me.
If you can afford the time, expenses, and effort & are experienced enough to take care of her and get her healthy, I would say go for it. :)
I got my horse Shamrock for $500, which is extremely cheap for a horse. He wasn't nearly as bad off as that horse looks like she is, but he has been the best investment my family has ever made (granted, my dad will say otherwise since he pays the board, farrier, and vet bills)!