Your rifle will be TOAST long before you go through 50lbs of powder. Besides, what if after using a couple of pounds you find a different powder that works better?
I'd limit my purchase to 8lbs and thats if you're fairly confident that you will be sticking with it. Large dealers can often give you several single pound cans from the same lot, if thats any help.
I keep my powder in its original container and have it in a cool, dry area. Some of my powder is many years old and is used periodically when working up loads for a new cartridge. Storing powder this way, I have never had a problem.
Just my 2 cents.