Mini's are basically a horse in a small body! A lean-to is just fine for a mini. I'm sure most barns would be plenty big enough for a mini. As long as it has a pasture to run around then the barn doesn't have to be really big. I'm not sure about tack, but I know it's really easy to find halters for minis. And if you want to teach them to drive (I'm doing that with my pony) then you can go to a tack store and you will probably find what you're looking for! Good Luck!