Mastiffs are a great breed. However, it is very important to do your research before getting the best dog you will ever have.
It is important to have them health tested. This mean having their hips and elbows certified by OFA or PennHip. Having them DNA tested clear of PRA (Progressive Retinal Atrophy). It is also important to have them CERF (Canine Eye Registration Foundation) cleared. Cystanuria, Von Willenbrands Disease, Thyroid, Heart, Patellas are all other health concerns. While there are many potential health problems associated with Mastiffs, health testing greatly diminishes your chance of getting a puppy that willl develop issues. Responsible and reputable breeders will health test all of their dogs as this is the only way to identify and erradicate the diseases from the breed.
Regardless of whether one wants to show or not, correct comformation and breed type are important. AKC papers should always be included with a puppy. The breeder can check either full registration (breeding and showing privlages) of limited registration (no breeding and no conformation showing). The papers are the heritage that belongs to that individual dog.
There are many wonderful people and places to learn about this great breed. First and foremost is the parent club, the Mastiff Club Of America (
http://mastiff.org/).
There are two wonderful forums that many of the top breeders are members of and post regularly. If you take the time to learn from them, it will be an endless source of information and knowledge. The 2 forums are the Mastiff Message Board (
http://mastiffs.org/) and the Mastiff Sweet Spot (
http://www.websitetoolbox.com/mb/acslinda)
Again, Mastiffs are truely a wonderful breed, but it is important to educate yourself about the breed before jumping in.