I'm sorry to have to jump on the bandwagon and give you more advice you don't want, but this is not a good idea!!
First off these big dogs make BIG messes. Are you ready to pay for the couch he eats that belongs to your roomie while you were in class? Or follow him around 24/7 to wipe up the slobber?
Second they need BIG wallets. Many people are shocked at the prices of well bred giant breeds but that's nothing compared to what it costs to raise them! I have an 8 month old Dane who probably costs me at least $200 a month, NOT including any vet care, training, etc. We're headed to the vet next week for a regular check up and summer prep, annual fecal etc etc and I expect it to cost me around $200. Meds cost a lot for big dogs - last month he had a reaction to a spider bite that cost us over $200. Emergencies such as bloat, broken bones, or illneses can run into the thousands. Even routine things like a neuter - minimum $100, spay is more.
Nor have I included the costs of training (absolute must). Each class runs about $150 on average, then what if you end up with special training needs? Tons of dogs have issues with big dogs - which can lead to problems. Luckily my boy and our bullmastiff buddy (who is here more often than not) are both too goofy to know, but my friend's English Mastiff has severe dog issues because he's been attacked so many times. So now they're dealing with a 200 lb dog that lunges and looks like he wants to eat any strange dog he sees and are paying a pretty penny for private training.
What about when college is over? Or on breaks? You can't really fly these dogs. The crates that most of them fit in (min $200 investment by the way) are too big for airlines. Flying them involves a lot of work and stress on the dog, and minimum $300. These aren't dogs you travel easily with. They also overheat fairly easily, especially Bullmastiffs, so flying could be even more a health hazard for them.
Are you aware of the special health issues with giant breeds? Generally they do require special care and consideration as far as diet, types of exercise etc, and also you have to watch because no matter how sweet and gentle, they ARE a liability. No matter what someone or some other dog did to my dog to provoke him, or any other large or giant breed, it's going to end up being blamed on your dog. You could be sued, your dog put down, etc. Even if you feel you have total control, you share your home and what if they accidentally let the dog out and he bites someone? It happens. Twice in the last two years I've had houseguests let my pets out by accident. They are not their animals, so they are not as diligent.
The other problem is that no rescue is going to adopt to you. The dogs come first, and too many giants are passed around because people are not as prepared as they think for these big guys. You must have a fenced yard in almost every case, and I don't know of any mastiff rescues that would adopt to someone who'd never had a giant breed before.
How about volunteering with a mastiff rescue, getting involved with breed clubs, etc and doing some more research first! I'm sorry to be so negative, and I totally know how hard waiting is...by the time my pup was ready to be born I was seriously psycho. I would wake up at night and scream at my husband - "where's my puppy, what did you do with him?!" - seriously!
I've wanted a Dane forever but didn't get mine until I was 25, owned my own home with fenced yard and had the time, money and support a dog needs. College isn't the best time for getting a dog, especially not one of these guys.
Please, I love these breeds a lot, and it breaks my heart to see so many in rescue. I know you mean well, but so do so many others who want a big dog, but along the line they realize they can't handle it, and it is the dogs who suffer.