For dogs that will sleep outdoors, you still have to provide a house for them, and use judgement. There is a broad range of dogs who can sleep outside, but even in this category there are some which are more naturally hardy than others. This does not mean the dog will necessarily guard the house, attack intruders, etc. Some can be trained for defense, or to subdue an attacker. Police dogs are used as guard dogs, but so are the large dogs used to protect the flock against wild animals.
Some breeds seem to require more time to train than others.
On the other hand, some dogs are naturally easier to train. Please consider how much you are willing to put into the job. There are breeds which respond to only one master. There are also breeds with a tendency to bite. Of course, these are tendencies, and not all, or even most, individuals would exhibit this trait.