I presume that you are familer with the color picker here http://www.nycewheels.com/brompton-colors.html (click on the color menus and see what happens).

I don't have a rear rack and I believe it is more difficult to roll. To do so you unfold the stem/handlerbar and use it to push/pull the bike. The problem is that the wheel base of the easy wheels is too narrow and you need to balance side to side. Also, on certain angles the rear suspension elastomer hits the ground. The rear rack also make the bike more stable when you Brompton (park) it on uneven surface. I don't plan to get a rack though since I don't need to roll much. The bike is simpler without the rack.

What handlebar type do you plan to get? The C and T bags do not fit S handlebar.

Edit: to the OP, note that Brompton is very good with spare parts availability so if you will change mind mind you can always add a rack.