I have nothing against outdoor dogs, as long as they are properly socialized and taken care of, but I think you will have difficulty getting one (or two) from a shelter or a rescue. I really don't know anything about your area, or the pet situation, but I know that here in Seattle no one would adopt a dog to a family that planned to house it outdoors. The simple fact is that a lot of dogs that are surrendered to shelters are outdoor dogs that are given up because the owner can't take care of them properly, or they were causing a nuisance in their outdoor home. Therefore, even if you did manage to adopt a dog that is used to living outdoors, you will probably get one that is dangerous (because it is not properly socialized, or has become barrier aggressive from being chained or penned up without proper care) or destructive and not a dog that you would want to own. To be clear, I am not at all saying that all outdoor dogs are dangerous; but many outdoor dogs that are dropped off at the shelter are. Our male german shepherd that we adopted was an outdoor dog that was surrendered because he was barking and snarling at strangers, other dogs, and children - this is because he was chained up in his yard and was bored and had become territorial. If we had taken him home and put him in a similar situation, with an outdoor living space, I expect we would have had a similar outcome. However, with extensive training, socialization, and a structured indoor home, he has become a very sweet and well-behaved companion.
We adopted a female german shepherd a week ago and she is on the other extreme. She was clearly someone's baby, and she follows me around the house. If I had taken her home and put her outside, she would be absolutely miserable. She looks like a "guard dog" but she really just wants someone to rub her belly. She gets along very well with our male shepherd, but they are not each other's "pack". Most dogs that are raised solely with humans will never really bond with other dogs as strongly as they bond with humans.
Anyway, my point is that your chances of getting a good outdoor "guard dog" from a shelter or rescue are very slim, even if you can find someone who is willing to adopt to you. In my opinion, you are better off purchasing two puppies (I'd recommend a brother and sister from the same litter - spay and neutering them as soon as you can, of course) from working lines, from a suitable working dog breed. All of my personal experience comes from shepherds, but I know that some lines of dogs are bred for outdoor work and some are bred as pets, and you're better off buying a working dog - it will be happier with its outdoor "job" and will likely be better suited for outdoor life.