Great Dane Puppy Wanted

We are looking for a Great Dane or similar large breed dog. Ideally somewhere between 6 - 18 months to act as a companion/guard for my husband and our shop. The dog needs to be able to adapt to family life as we have three small children. But the primary rold for the dog is to accompany my husband to and from work and act as a deterrent to muggers. We have a family dog at home and are really looking for a large but docile, friendly pet. Just needs to look enough of a deterrent, although a calm temperament is fundamental.