If these dogs are not to be active family members I think you should consider one of the molosser guard breeds. A friend has a Boerbel that is a formidable property protector. The GSD is a herding dog bred to work and interact fully with its family, not a guard dog.
Molossers or Livestock Guardians were bred to work independently, such as you describe.
They aren't a "needy" breed like GSDs.