Another vote for the split hood coat. We have it in brown. BUT we moved here from the Rockies, and it wouldn't be enough for mountain or prairie winters.
As for boots, we found the UltraPaws brand to fit best and stay on. Muttluks are okay, but bulky. They are cute, but UltraPaws are much better all around. The dogs agree. Still not good enough for Rockies winter.
My honest suggestion for winter fun is a good coat and booties... And a backpack or saddlebag with warm bedding for when the pup has had enough. You know yourself and your horses best in terms of safety, though.