My barn has an enclosed yard, so the horses are turned out and in according to paddock, and go out(and in) on there own, with someone going to open/close the paddock gates, and someone opening/closing stall doors. It makes it go a lot quicker, but it is a rather large barn.
Lockers are a nice touch if you can afford to do them, and they really don't have to be huge.
You don't have to have a heated barn by the way. If you insulate it well enough and the correct way, it should stay warm in the winter and cool in the summer.
Just as a sidenote....you should price out the cost of a barn kit versus the price of buying the supplies. It could be possible that one over the other could be cheaper.