Agree with most of what tempest has said - mastiffs are my breed of choice, too. But, as per what others have said, be aware of the food bill, the training requirements (which are ongoing, not just puppy school
), the vet bills down the track for joint issues, and also how long you will have a puppy for!
The bigger the dog, generally the longer they take to grow out of the puppy stage. My boy was 2.5years before he started to calm down, and 50-60kg of puppy is a challenge! Just something to keep in mind.
And, the vet costs. We are currently just starting down the arthritis pathway, and it's expensive. Change of diet to $150 per 2-3 weeks per dog, plus monthly cartrophen injections, plus xrays... definitely get insurance! He's 4.5 years old, by the way. We would expect to get another 3-4 years out of him, and I'm already preparing myself for it. It will be worse than losing a limb.
But, otherwise, great dogs
And, even though they're big, they have great temperaments (usually!) which makes them suitable as a first dog, so long as you're prepared to train them and put the time in
Good luck!