Personal protection dog?

[Daigga]

So my husband and I are expecting our first child here near the end of the month which has gotten me thinking lately about home security and ways of keeping our family safe. To be perfectly honest, I'm just really uncomfortable with the idea of mixing guns and infants at this point and the more I consider it the more I would prefer a well trained dog. I've owned protective dogs before, but never an actual personal protection dog, so I have lots of questions about if this is a valid option for our lifestyle or not.

First thing is that we live in an apartment at the moment. I'd like to make a move into a house, but the earliest that could happen would be July of this year and there's no guarantee that it will happen then at all, so it may be apartment living for a good deal into the future. Second is that we have a dog right now already and he is definitely a permanent fixture in our lives. Odin is a sheltie border collie mix who likes to bark and is crazy smart, but he's no guard dog and I really doubt anything can change that. He gets along great with other dogs, kids, cats, and everything else, but should I expect problems with adding a personal protection dog into the house? I had honestly considered another dog anyway just so Odin could have a playmate whike my husband and I were occupied with our newborn (he's the spoiled rotten center of attention at this point). With my current dog, a newborn on the way, and apartment living, is a personal protection dog a good choice for me?

If so, what would I be looking for? I'm not picky about breed and wouldn't mind a mix. Should I just keep an eye on what the local shelters have up for adoption, or should I be looking into breeders? What about age? Do I absolutely want a puppy to raise up with a close attachment to my newborn and my family, or would I be just fine getting something around a year or two old, because I would love to skip the potty training/teething stage! Are there certain personality aspects I should be keeping an eye out for, and what would those look like on an untrained dog? Lastly, what can I expect a good trainer to charge? If I pursue this price wouldn't be a deciding factor, but I would still like to know about what to expect.

Any advice is appreciated!