All of the windows on the lower level need replaced, new doors all around, paint? I really think the decision on function of the barn needs to be worked out. I'd hate to spend a tremendous amount of money and time fixing it up with no real purpose in mind.
A "people" living space would be great, as well as hay storage. I would also love an indoor area to groom horses in the winter or rainy season. I do not desire to keep horses inside unless it's an emergency.