I think it is very hard to judge just what future buyers will or won’t like about your property. Unless you are moving very soon I would do what makes you happy and enhances your enjoyment of your property. I get that a multipurpose building may appeal to more people, but personally I would prefer that cute little shedrow barn to a hulking metal building if I were your prospective buyer. That goes double if the barn will be the first thing you see on the property and something that is highly visible.
In my area small horses farms are hard to come by and quite valuable. I wouldn’t consider a makeshift barn inside a greenhouse to be a true barn, so if I were your prospective buyer I would want a real barn. I would also personally really love a greenhouse.