We have one. DH built it when he built the house. Concrete floor, drain, surrounded by rough cut lumber with a rough cut lumber shower door. (looks like a gate). While the boys were growing up it was used daily. (They were always, messy, muddy). Now its where the dog gets his bath. But mostly it just stays full of leaves, spiders, and other crawly creatures.
My suggestion would be to use material that does not have to be cleaned and scrubbed. The wood weathered naturally and the concrete floor can just be raked clear of leaves, spray a little bleach, rinse and its good as new. In 17 years this one has had no maintenance at all. Even the shower head (cheap-o from Wal-Mart) still works. Its attached to the house, (backs up to the Master Bath so hot and cold was not a problem).
My other suggestion would be to consider how often you would use it. If you do have kids, its just one more thing that has to be check and kind of cleaned occasionally. Another pipe to burst in cold weather, another leak waiting for a place to happen. Not to mention the spiders, snakes and other crawlies that think it makes a really nice enclosed, protected living space.
If its not something you would use on a regular basis, and you want hot and cold running water, make something temporary using a water hose that you can roll up during the winter months.