On a couple of the breeds suggested, Boxers are great dogs but have a VERY high energy level and may not be the best if you don't have room and time for them to run. German shepard dogs have been essentially ruined by the show breeders. In an attempt to get a "look" that the judges like they've changed the breeds structure so much that the vast majority of them end up with dysplasia. If you want a shepard type dog look at a Belgian Malinois. They are still healthy and that's what you see a lot of schutzhund people switching to. The American Mastiff is still a bandog, and despite what breeders say does still drool and suffer from dysplasia. I have a Cane Corso and they "don't drool" either, though I think that if you ask anyone who owns one you'll find that that's not exactly true. Large dog will always have a chance at dysplasia, by breeding dogs who have good hips all you are doing is reducing the chances for the pups to suffer from it as well. As far as life span goes, the bigger the dog, the shorter the life. Selective breeding and better nutrition have changed that a lot in recent years. A decade ago the average life of a Great Dane was only about 6 years, now its around 10. You still won't see too many making it to 15 though, it just the nature of the beast. As far as the molosserdogs website, they don't exactly have the best reputation in the dog world, Try this site for better info
http://www.moloss.com/A pet quality bullmastiff isn't any more expensive then a pet quality rottie or any other large breed dog, you're looking at an average of $1000-$1500 for one that's well bred, unless of course you choose to rescue then you're looking at considerably less and you're doing a good deed at the same time.
http://www.petfinder.com/ English Mastiffs are awesome dogs but an average male weighs about 60 pounts more then an average bullmastiff, and about 80 pounds more then a rottie. Neopolitans are also beautiful but I really wouldn't recomend one for a 1st time dog owner. A properly bred Neo can be a serious handful for the inexperienced. Beagles, while cute, fall into the catagory known as "feeder dogs" fun to play with, good for hunting with; but they don't deliver a whole lot of peace of mind if they're what's protecting your loved ones while you're away. One other breed you might want to look at is the Dogue de Bordeaux. Its the dog from Turner and Hooch. Great family dogs. About the same excersize requirements and size as a Bullie, and great with their kids. They don't drool quite as much as Hooch did in the movie, they fed him a constant diet of honey to keep the juices flowing. What area of the country are you in? There are a good number of places with breed specific legislation now and that may also have an effect on your final choice. Any more questions, post em here. I'll be on for a couple more hours before I have to go out and celebrate my heritage. Cheers,
Shawn