There are different options Ian. Personally, i'm going with a lot of goat milk powder, pretty nutritionally dense, easy to digest, and fairly light weight and easily kept long term.
You can also go with dried whey, but it will have less calories/fat than the goat milk powder.
I don't recommend cow milk powder because of how hard to digest it is. When a food is hard to digest, it takes more energy than is helpful from your body to try to break it down.
Whey on the other hand, is all the good stuff in cow milk, without any or very little of, the bad stuff like the gluey casein proteins or lactose. Some wheys have more lactose than others, but for someone who can't drink cows milk without getting noticeably mucous'y, i can drink goat's milk without a problem. Yet, they both have lactose--the difference is that the goat milk proteins are much smaller and more easily broke down than the cow casein proteins.
It's the casein proteins that make the "curds" in that old nursery rhyme about little miss muffet, all the rest is the nice, nutritious and easily digested whey.