There are plenty of dogs out there who are in need of a home so why not adopt one? Although you are playing the lottery on what their looks and traits will be when they are fully grown, compared to pure breeds, I think that part is what really makes it fun and rewarding. But i guess you are looking for guard dogs.
For security I find dogs with a keen sense of smell/hearing(typically smaller dogs), who run to their owners or keep a distance from the door and bark better than dogs that just run to the door/gate and bark. Criminals are not stupid and when they encounter dogs at doors or gates, they'll spray pepper spray or some toxic aerosol(hair spray is popuplar) onto their face through the gaps in order to incapaciate them, if not kill them(a method on the rise here in Argentina). Then they are useless and you've just lost a pet that you love.