There's a lot more to rescuing a horse that putting groceries into it
Sometimes the things that don't show up at the time, can be more grief and cost than getting the horse to good weight.
I could be wrong and I hope it's the camera angle, but this horse looks ewe-necked to me. Not good if that is the case.
Ewe-necked can be too short or too long of a neck on too straight of shoulders, and also looks like it was put on upside down.
If you Google "ewe necked horse" you will get some credible hits; here is one. Icelandic Horse, Ewe Neck
Given that he may also have a club hoof, if you want the horse merely to save him and don't have plans for anything more than a bit of light hacking, he might be worth the risk if he is a kind-natured horse.
$400 is a lot of money and the horse possibly could be even more heartache in the end. It would be wise to have a vet examine him
Wish I could say more but, were it me, I would most likely swallow past the lump in throat and walk the other way, IF the horse really is ewe-necked. You need guidance from a vet:)
And yes, another inquiring mind wants to know why the BO let this horse get in this condition? I can think of a couple possibilities, in all fairness to the BO, but I am still curious