I currently have 2 male presas. My first bit of advice would be to research the breed thoroughly before you commit yourself.
The breed is a working class breed that requires physical and mental stimulation. If you don't have the time invest in their training, socialization, and exercise you may end up with a dangerous situation on your hands.
The breed is very dog aggressive and if not properly socialized as a puppy you'll run into problems. They need to be exposed to a variety of people and situations. We even introduced our dogs to various objects. They investigate anything new in their property. That seems to be the reason they are good guard dogs.
Obedience classes are pretty much a requirement, both for socialization and to be able to control a 100-160 lb dog.
If this is your first experience with a large breed dog I would probably recommend against it. You have to be very assertive and be the "top dog" in the pack otherwise the dog will take the position. They are very stubborn almost to a fault. Another reason they are probably good for guarding, I don't think mine would back down against anything. If they are not trained and don't know you are the dominant figure they will not back down against you if they really want something.
If you still think you are up for the challenge, they are excellent guard dogs of anything they feel is "theirs" (house, yard, family). They can read us very well and if we aren't comfortable with someone they can be aggressive, but if they can tell we are friendly with the visitors they will be very friendly. Lastly they seem to think they are lap-dogs and will do whatever they can to be as close to you as possible. They are very loyal to their owners and expect to have a lot of interaction, I don't think they would do well as a strictly "backyard dog".
Sorry for the long post. This is a great breed if you put in the work. If you just want "big mean looking dog" I would look for something else.