Tazman,
I have heard good things from folks with back injuries who ride the gaited horses you mentioned. Others would be the Paso Fina(sp) or the Peruvian Paso. Most mules also have a smoother gait than a horse, but not as smooth as the gaited horses.
As far as using a horse out West; the horse don't know where it is at, but you will want trained to be ok with high places and river crossings.
It should also be shod properly for the sharp rocks out there; maybe some pads on the front feet if the area has really sharp rocks. I like dark hooves the best as I think they stand up to abuse better than a light one.
You also don't wan't the horse to freak out at the sound of a rattler. They should be ok with ropes around their legs. You just want them steady in general as you are a long ways from nowhere out there. It would also be nice if it didn't run off when you fired your gun.
Happy horse hunting.
Hud