See this is all a hard no for me. Why buy someone else’s problem? Are you buying from a trainer? Then they have him tuned up as good as he’s ever going to be. Are you buying from a backyard ammie? Then he could have a million things wrong
Sound is the basic baseline for buying a horse.
You don’t get a PPE on a visibly unsound horse. You get a PPE on a horse that looks sound to your eye but you want more certainty.
On a horse like you describe, you get diagnostics and a treatment plan. You already know he is unsound and not fit to ride. Why take this on if you don’t have to?
Back problems are notoriously hard to fix and won’t resolve in a week unless he gets some medical help (undisclosed or disclosed) from the seller
Walk on this one. You are in for a world of heart ache.