Different breeds will have certain characteristics for which they were bred. Ask breeders at dog shows and look them up in breed books for further information.
Dogs are social creatures.
They will not be happy left out in the back yard alone. You must be committed to spending several hours a day with it. Conversely, many Mastiffs are content to flop on the floor and do nothing at all while you are gone. Even if you get a dog from the shelter or otherwise inexpensively, you will have to buy food, pay for veterinary checkups, vaccinations and routine medical care, and purchase other equipment over the lifetime of the dog. Not to mention replacing anything the dog may damage or destroy, or putting money out for medical emergencies.