Been there, done that, a number of times. You want a sturdy, settled horse of some size. Good feet and legs under him. I also believe that the workhorse breeds are too large. Quarter horses seem to work out very well for this duty but standard breds are just as good depending upon the animal. Have you considered a pack mule? On one trip I rode a mustang and we packed with him as well. He wasn't very big but he was tough as nails. Your size also dictates the size of the horse you want. The bigger you are the bigger your horse should be. You are getting into something that is generally not for the novice. Horse packing in the mountains takes skill, MONEY and a lot of horse sense. You need to have a thorough knowledge of horses and their care. You probably need to have professional training in loading, securing and balancing a pack animal. My advice is to go out with a good packer and milk him for all the advice and knowledge you can. Packing horses in the mountains is an art and there are very few left with the knowledge to do it properly.