Welcome to the forum!
In reverse order, I really like my Ortliebs; their Sports Packer (Plus) should carry 15 pounds with no problems.
That's not a terribly heavy load, so I suspect you could carry it on a rear rack. Depending on the bike you'll be riding, that might be easiest. Some bikes, like those with a carbon fork, can carry a rear rack, but not a front rack. You might even be able to carry your gear in a large saddle back (like some of the classic English bags wallbike.com carries).
The major advantage of front racks I can think of would be to allow you to carry extra water and food in a rear trunk bag, as well as a jacket, camera, etc. The BRP has some long stretches with limited services. The Sports Packers will fit either front or rear racks.