I don't think it would put me off particularly but I've bought several project houses.
The pps idea for the animal hydrotherapy is genius and that does sound like a perfect solution as that building is not well insulated or in good nick, it's basically a shed.
The thing is you can't just leave it as it is really. You would either need to spend 10-20k at least to up-spec it. Or pay to fill in/remove.
If I were interested in the house I would put in a lower offer to reflect the cost of one or the other depending on what I was planning. Tbh it needs a whole new building.
I can see however that some people who don't want a project would be put off.