Boy oh boy I understand the fear of not making a common sense decision if you fall in love with the horse. None of my horses were purchased with common sense.
A big factor for me would be your knowledge of farriery and your access to quality farrier care. Coffin bone issues are going to be dependent on how well the feet are managed. Bad farrier work is practically ubiquitous in the horse world. How close are you to a university with a podiatry program?
If your farrier is already saying no, that’s not a good sign for how this will all work out in the long run.
With that said, you don’t ride x-rays. If the the horse isn’t haven’t any issues now, this may not even be an issue. The lousy thing about PPEs is the are only a small part of the picture and no one has a crystal ball.
Personally, if I was in the same situation, I’d probably go look at the horse if I really loved everything else about him. (I’d also probably be an idiot and talk myself into buying him even if it was a bad decision, so take this with a grain of salt)