You could always do what I did.....Go to the pound and find a slightly aggressive, not likely to be adopted dog. If you take the time to work with them and provide them a good home they'll be great pets. As for a guard dog...you already know their instincts.
I have a lab hound mix. Picked him up at the pound. He was scheduled for euthanasia That day. Makes the best family dog I've ever had but Will Not hesitate to tear into a stranger unless I say their ok. I know this because he's already done it
Adopted a rottie/shep mix too. She was used for dog fighting and was also scheduled for euthanasia. Now she's a great big loud barking baby that never leaves my side.