I've dealt with mastiffs, and I have a few friends that own them.
They are BIG.
They eat a LOT. They drool a LOT.
Be sure you can provide tons of exercise. Letting one into a small yard is not enough exercise. Be aware of the health issues, like bad hips, bad knees etc. Get yours from a acreditted breeder, who can show you documented good hips on the ADULT breeding dogs. Any puppy can get a clear cert, go for the adults.
Don't let them get overweight. Do train them early. Do keep them properly contained. Do be sure you have the space, time and mentality to deal with a large(huge) unruly dog.
Wolfy