Barn Re-Roof

My wife and I purchased an early '70's desert ranch style home in the Spring of 2012. With it came a barn that leaks horribly! One end of the barn has a concrete pad "tack room" and the other end has 2 horse stalls. The roof is nearly flat and slopes from West to East. (West is to the left in the picture) There is 12 inches of run and maybe 1/4" of rise across the full 50+ feet of the barn. Like I said, nearly flat. It has plywood over the industrial steel trusses that make up the ceiling structure which are supported on the East and West ends and in the middle of the barn by industrial steel I-Beams. Above the plywood is your standard corrugated metal. There are 1 foot overlaps with the metal and they are held down by washered screws. All of the siding, some of the 2x4 walls and all of the internal electrical needs to be replaced as well. Every pack rat, termite or critter that chews or pecks has destroyed most everything in there. I'm figuring over the Christmas break would be a great time to do it. I've got nearly 3 weeks of time off. The hard part is the $$$. As I've been looking at metal roofing products, everything I've seen says that I have to have a minimum of 2.5" of rise to 12" of run. Is this truly the case? My intention was to peel off the corrugated metal and the trashed ply. Then put down new 1/2" osb followed by a fiberglass roll then by all new metal with the proper edge overlaps. Is a Metal Roof not the right material or would you recommend that I switch to a normal built up roof? Something like Plywood followed by a fiberglass roll followed by white coat over that? Thanks in advance for any ideas / suggestions you may have.

+1 Happy Thanksgiving by the way. I would run a mechanically seamed standing seam as Todd mentions in conjunction with some small sloped ISO insulation. This would give you some of that minimum slope that you require and help the roof shed the water more effectively. I don't like the membrane roofs in the Southwest/desert climates given the harshness of the sun and the potential for UV degradation. Todd is correct that they would be indicated for the pitch that you have in this case as well but I just tend to think they burn up to easy in the sun without some sort of protection of ballasted installation.