The problem with "guard dog training" is that it's hard to measure what's reputable and what's not. Cost is not an indicator of quality training.
The plus of a WELL TRAINED guard dog who's trained to attack is that he's also trained to STOP when commanded to do so. A loving pet, thinking for himself, is less likely to be quite as under control. So while you can compare an untrained or poorly trained "attack dog" to a loaded gun, a well trained attack dog is a bullet you can call back.
Boxers and Rottweilers tend to excel at a certain kind of guarding -- mainly holding. In holding, they will let an intruder IN, but not let him leave! It's not necessarily a vicious thing, either, just sheer intimidation. More than one innocent workman has reentered a house to get a tool and been confronted by the formerly friendly family pet who is determined to keep him there until the owner says he can leave.
They are thinking and guarding dogs -- that's sufficient for most needs. There is a test given by the American Temperament Testing Society (ATTS) that measures protectiveness, among other traits. If you seek their website you may find a test near you. This provides the dog with a series of challenges in a controlled setting (strange noises, including a gunshot and rocks in a metal bucket, walking on peculiar surfaces, and greeting a series of strangers from totally neutral to friendly to peculiar, to threatening. The whole test is done on a 6 foot lead, with no handler interaction (other than holding on tight).) The dog's reactions are carefully noted and scored against his breed -- obviously you want a different reaction from a working dog than a toy dog.
The neutral stranger just exchanges pleasantries with the person and ignores the dog totally. The friendly stranger greets the dog as well. The peculiar stranger is dressed in some outlandish fashion but quite a ways away. Then the peculiar stranger comes closer, and slowly becomes more threatening (never getting closer than the leash length!) Threatening is usually yelling and waving a stick around.
It's a decent way to gauge a dog's reactions. I'll never forget the fire of a little french bulldog when the threatening stranger approached -- it's not all about size, but heart.
Don't do these tests yourself, though -- you want it done in a controlled environment with people who know precisely how far to go. But if you truly wonder if the dog would step up in an emergency, it might provide your answer.