Agree with the rack. It's actually pretty light (we fitted one to Mr Houtkop's bike, as I bought a M6L for him rather than the M6R we wanted because it was in stock & I'd heard of the waiting times - see eg Charlie's post above).
Additionally, the drag when using the rear bag is vastly better than when using front luggage. My commute seems to have a permanent headwind when heading home (no tail wind when heading in though, unfortunately!) so I've taken to using the rear rack only whenever possible. I do also go shopping with my bike though, and then I need the front basket, rear rack and backpack!
I would however not recommend the dynamo lighting (tyre) - can't comment on the SON as I don't have it.
Also a good upgrade is to get Ergon grips. Just upgraded mine and I think the new ones are great! This isn't a Brompton option so you'd have to do it post purchase.