First off, congratulations and welcome to the obsession!
Second, I would MUCH more than 2lbs of brown sugar. I just finished a 5gal batch of hard cider and I used 2lbs of brown sugar in that along with Nottingham's, so a very similar recipe to what you're using. If you're doing 17gal, I would recommend something more like 5-10lbs of brown sugar if you want any hope of getting a residual taste from the added sugar/molasses.
That being said, you don't HAVE to add any sugar at all. Adding sugar bumps ups your ABV by giving the yeasties more food to much on, and to convert to alcohol. Adding specialty sugars like brown sugar, turbinado, cane, honey, etc. *CAN* add some subtle flavor profiles depending on the amount used. Also, you can use regular dextrose (corn sugar) which will ferment completely and leave no added flavor, if that's what you're going for.
I hope you plan on kegging that massive batch, I'd hate to have to bottle 17gal!! haha
Either way, have fun, enjoy your brew!