How about a livestock guardian like a Pyrenees? They're not extremely high energy dogs (the adults that I know are laid back, but still respond quickly to a potential threat) but they're naturally quite protective and working-bred Pyrs don't need to be trained to do so - though obedience training to keep them under control is a MUST.
But either way, getting the dog will be the smallest part of the equation - the big deal will be you and your dad learning how to handle the dog.