I fourth (fifth?) the bathroom idea. Although I do not have my own horse, I still go to the barn straight from work for lessons sometimes, and use the bathroom to change in. I also normally have to pee before or after a lesson, and would be ok to pee in a stall...but there are too many people there all the time to do it comfortably.
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.