Virgil's right about the PVC80.
Be sure to run everything outside of the stalls away from the horses as high and tight to the structure as you can. Any piece of pipe that can be used for hanging tack will be, and any spot that a rope could catch on will catch one eventually. Try to stay away from any wood that will have to be replaced due to cribbing or rotting.
Put a switch for the lights in an easily accessible spot far away from the horses for use in those "night call" emergencies. Use enough lights that come on instantly (Qz, etc) that you can see before you go in. HID's are Ok for general lighting, but take too long to start in an emergency.
Try to mount your lights in a way that vibrations from the stalls won't be transmitted to the lamps or you'll be replacing lamps way too often.
Don't forget ceiling mounted recps. for fly zappers. If there's a chute that will be used for doctoring/grooming, put a GFI recp close enough to plug in clippers, etc.
(And if it's a stud pony, quadruple the "crib/tantrum factor" [Linked Image]) All horses are on self destruct mode 24/7, whether they know it or not.