Shouldn't you educate yourself first?
"hey, get a mastiff! You cant handle a small dog, but get a huge one, this way, when your dog attacks the neighbors dog again, you won't have to worry about the neighbors dog cause it won't be alive!"
Also, it's a known fact that larger breeds have more health issues.
The OP's lease ends in October, and they cannot afford to move out early. If they can't afford to break their lease (or double pay rent until it ends) to keep their previous dog, than how in the world will they be able to afford a (very) expensive puppy? Heck, we spend over $100 a month feeding 55lbs of dogs. English Mastiffs weigh
120-250lbs... I can only begin to imagine the cost in food!
Let's take a rough guess and say that an English Mastiff (or any purebred dog, really), is going to cost over $1000 from a responsible breeder. Factor in vaccines, spay/neuter, and the cost of dog food over a year. We're looking at at least $2000 in costs. Throw in an emergency vet visit for bloat and we're close to $5000 in costs.
Most apartment complexes have size limitations as well. Around here, it's common for them to only allow dogs under 25 or 50lbs. Can't really sneak an 150lb dog around a complex where they're not allowed.
What a stellar idea.
I'll also recommend the pet rock. Maybe a pebble sized one that can be carried around in a pocket, to ensure it won't get loose and clobber the other rocks in the apartment complex.