Protection dog training for newbs

This aspect of dog training seems really interesting. I'm getting my dog pretty good at basic obedience, sit, stay, heel, down, retrieval, kennel, and the like. I see a lot of bite work pics here, and it seems like this could be a good direction to take my dog.

I have a few questions. Perhaps this thread should be stickied.

First, what exactly makes or breaks a dog for this line of work? I ask, because there are those of us so ambitious to take a dog from the SPCA and start training from scratch and see where we can stop. I love challenges... I'm a homebrewer, comedian, musician,

Obviously, one would imagine a strong love for its owner, high energy level, strong jaws, enough muscle to anchor those jaws, the "game" attitude of 'yea, I see a challenge, let's do this' but after that, what qualities will or will not be in a dog cut out for this line of work? Is a quiet dog a good protector? What about a dog that barks a lot?

Is it typically difficult to find great trainers for this type of work (obviously this is dependent on locality, but generally speaking)? How much time and money would one be willing to fork down to satisfactorily get a dog ready to protect a home with any real degree of success? How much up-keep is there, training-wise to keep a dog ready?

What tips, suggestions, comments, criticisms or concerns do you have about my general premise?