My suggestion:

Buy the 6 speed. The amount of weight added as well as the cost is a good cost/benefit investment.

Buy "a la carte" without the rear rack (or the below-average light system as a matter of fact). With the front block you can have all the stuff you need in a more balanced scenario, attached to the front of your bike. If you really want a Brompton rear rack, message me. I have one here that I used once. Will be more than happy to find a way to have it reaching you.

14R