My mother's boyfriend breeds bulldogs (english) and bullmastiffs. I grew up around these dogs, so even though I don't have one right now I can give you the skinny.
The trouble with bulldogs is they're highly individual. There aren't really any breed traits when it comes to the dog's personality. Some of them are calm, some are hyper. Some are aggressive, some are lazy.
They're pretty low maintenance as far as dogs go; you walk them, feed them and they're pretty much okay. They do like to hang out with people, but they're not the sort of social dogs like, say, retrievers or a lot of small breeds; they enjoy your company but aren't dependent on it.
My biggest caution when it comes to bulldogs is not to underestimate them. They're not very tall at the shoulder, but they have a lot of mass. They're capable of knocking a teenager or small adult on his/her ass; they also can get a bit rough when they're playing. If you have small kids or other small dogs, this may not be the breed for you.
They're also quite possessive, especially when it comes to food. Do not, under any circumstances, sneak up on a bulldog that's feeding. It's a bad idea.
In terms of health, they're not the most hearty breeds. Jaw and respiratory problems are common and they have trouble with hot/humid weather. I've known bulldogs that have succumbed to the heat on long car trips. If you don't want this to be your (very expensive) dog, make sure that you have air conditioning available in this sort of weather.
Bullmastiffs, on the other hand, tend to be very calm animals. They're also quite intimidating and make good guard dogs; while generally harmless, they do have a powerful bark that they're not afraid to use. They're not overly aggressive, however. I've seen a few nips, but I've never seen a bullmastiff ever get really worked up.
They are big dogs and they're lazy as all get out. They like company, but again aren't overly dependent on it. If you're taking a day trip, the dog will be fine in the yard, so long as she has a source of water and some food.
A bullmastiff is a good dog for families. They're generally very tolerant of children and small animals, especially if they're raised in that environment. They're the sort of dog that will stretch out on the living room floor while you're watching a movie (usually on your foot).
Bullmastiffs sometimes have respiratory problems and hip trouble is quite common among the breed, as it is in most large breeds. For the most part they're okay, although they're not especially long lived. Typical lifespan for this sort of dog is about five years - eight is getting pretty old.
In terms of purebred verus shelter dogs, purebreds do have some advantages. They tend to be more predictable, because you know the breeder, the breed and the environment. There are fewer variables in the dog's upbringing.
Purebreds are, however, prone to many more health problems than halfbreeds or mutts. That's mostly because of the in-breeding that went on to make them the way they are. Defects among a pedigree are often passed on, so if you get a purebred dog of any breed make sure you meet at least one of the parents (preferably both, although that's not always possible). Talk to the breeder about the line's history. Many breeders will have been breeding that line for several generations and will be very candid about any potential health troubles. It's in a breeder's best interest to make sure you know exactly the sort of dog you're getting, as well as to raise good dogs and make sure they go to good homes. A good breeder will interview you before even agreeing to sell you a pup and should never sell a pup less than six weeks old. If you're offered a younger dog, start looking elsewhere.
And final notes on the sort of commitment you're getting into. I know you say you've done your research, but make absolutely sure that this is something you want to do. A dog is a huge commitment and will have an impact on your wallet as well as your life. You need to be prepared to walk the dog
every day; there is no missing a day, no matter the circumstances. If you're indisposed, you need to make alternate arrangements, whether that is letting the dog out into the yard or having your wife or a friend or whoever else do it for you. You also can't go on any sort of a trip without making arrangements for the dog.
Dogs don't care if you're sick, if you're busy, if you're tired. They don't care if you have better things to do or if the deadline on that presentation is tomorrow morning and you just
have to get it done. Their needs are consistent and they won't make an exception. Effctively, when you get a dog you have to include that dog as a factor in your schedule for as long as she lives. You give up a lot of freedom when you get a dog. The reward is worth it, but only if you're in a position where you're willing and able to put that sort of effort into raising and caring for the animal.
In a lot of ways, having a dog is like having a child, only this child never grows up. Until the day she dies, she will be getting into things she shouldn't, making messes, puking on your shoes, having the occasional 'accident'... you need to be prepared for all of this. I'm not trying to scare you away, but it's not something to be considered lightly. Having grown up in a household where dogs are bred and sold, I've seen my fair share of familys that are absolutely in love with the idea of Fido, but have no grasp of what the reality of owning a dog is like. Most of them left empty-handed.
Pictures stolen from the internets, as I don't have any pictures of the dogs from our kennels.