I'd just get on the bike and ride. You'll quickly figure out what feels right to you. Each person is different.
Load your bike up and see how it feels. What kinds of hills do you have planned?
For me: (28-44-50) (12-28) would be way too low gearing for all but the most extreme riding.
But, I suppose it never hurts to have a couple "escape gears".
One other thing. That looks like one of the basic Blackburn racks. Great, sturdy racks. However, at least the old style panniers tend to get a little floppy on the rear which can be problematic. Just make sure your panniers aren't hitting the spokes.