This is a good question.
There are things I don't notice (waterproof panniers, for example), that would be indispensable. I've had non-waterproof, too, and they are a pain.
... without a comfortable all day saddle under you, your ass is grass
In terms of the cockpit, I would go with the B17, too.
The other thing that is super, super, super useful is a handy change of shirt, preferably to something that does not hold water (like nylon or equivalent).
My $0.02...........