We have had 3 Danes. The best guard thing we ever had from them was their size & bark. They tend to intimidate most people but ours were not mean. Ours we very comical and big babies. I think I have mentioned how dh says I coddle them too much.
We decided against ever getting another one due to their short life spans & soooo many health issues associated with some breeds (hips, joints, cancer, bloat, etc). I can get on my soapbox when it comes to AKC, popular breeds, line breeding, etc.
Here's what I would consider: how old is it, what sex (will it mix well with yours), has it had any basic obedience training, is it house trained (in my mind they are not outside type dogs), any health issues (they can cost a small fortune) and is it an Alpha type dog to name a few.