Well, we had an old horse box/van (I don't know what you called them over the
pond) for transporting 2 horses. Very sturdy, spacious and waterproof. Since we no longer have horses or donkeys, it lingered sadly on the edge of the property for the last 4 years. Two months ago, we transformed it into an eminently road worthy chicken coop. Just made some perches and egg laying boxes and our chickens love their new home while we love the easy transportation from one field to another.
We have a Pyrenean mountain dog and he is an absolute cracker, however, it's more to do with it's nature than it's breed. For instance, his dad was an absolute pain in the what's it. By all means get an LGD, but find one that you relate to and you feel comfortable with, whatever it's breed.
Good luck with your endeavour, keep us posted!