To be honest mate, if you're after an intelligent dog used for guarding purposes, i'd not look at a Lab at all. Don't get me wrong, they are an intelligent species (if a tad goofy at times) but they've basically had most of their natural aggression bred out of them for years - although they would be more than capable of defending you if they knew you were in danger, they're not the breed most suitable for that!
Personally, if it were me, I'd probably be looking at getting an Alsation or two - they're also intelligent, you can teach them a LOT of commands which they'll learn quite quickly, and they also make much better guard dogs - they're bred for guarding so it's very much in their nature, and being naturally very light sleepers very much helps that. My uncle has always had a pair of them, so I've spent quite a lot of time around them! They're not that far off labradors size wise usually (although he has one at the moment that is a BEAST of a dog), and although they are a pretty independent breed, train them right and let them know who's the boss, and they'll be as good as gold.
The main method used to take out dogs is to offer a padded arm, the dog goes for that, and then a hammer to the head is used. If you've got two dogs, that isn't an option and I personally wouldn't see many potential thieves even trying!
Other options might be a Rottweiler, or if you've the space you could go for a larger dog? They might well be bigger and stronger, but train them right and that's essentially a moot point.
Couple larger breeds that spring to mind are the Pyrenean Mountain dog, Swiss Mountain dog, Boerboel and the Moscow guard dog. They're all about the 70ish kilo mark (for dogs, bitches are around 50ish), and all have to some degree guarding bred into them (the Boerboel and Moscow dog in particular). One of the most important things with those particular guarding orientated breeds is to make sure that they are very well socialised with both humans and other dogs whilst they're young, and if you bring strangers home, make sure you introduce them properly to the dog, else it might think you need protecting!