What you may do is contact the ranches that have herds of 100 or more that have that type of terrain on their property. When the horses that are roaming in the mountains as colts with their family herd, they develop natural "feet" and seem to negotiate mountainous trails much better than any "Bred" horse out there. A lot of your "Breeds" you mention are great for regular trails but if you've been thrown on your ass by one just because the wind blew a little hard and a branch or two made weird noises, you know what I mean. Hope you find what you need since theyr're like owning 30' boats, a big hole you throw money into.