2 or 6 ?
2 is best regarded as a single-speed with a bonus extra gear that doesn't cost any real weight.
6 if you actually need gears. Or buy a single-speed and retrofit a 8 or more speed hub at vast expense.
It mostly depends on whether you're optimising for weight of the folded bike, or ability to ride up hills [without sweating like a cyclist]. The Brompton drivetrain is a crud-magnet and the derailleur is the first thing to suffer. There's merit to a hub-only solution on this basis.
Kojaks or marathons ?
I went for the Brompton ones, which they've now retired in favour of non-plus Marathons. It's a reasonable compromise. Kojaks aren't especially fast (but grip nicely on tarmac), and are a liability if you ever do any off-road at all.
Big plastic luggage block on front or rack ?
Definitely get the luggage block, even if you don't plan to use it. It's cheaper to get it with the bike than as a retrofit, and the front luggage is one of the cleverest things about the Brompton.
The rear rack is best regarded as a stability enhancement for the folded bike with added mudguard protection. It means you can flip the wheel under in lieu of a kickstand, and makes the easy wheels work better (particularly important for shorter riders who can't carry the folded bike with a straight arm). Rear rack luggage can't be trolleyed around on the folded bike like front luggage can, and the rack is too low down for panniers. I use it as overspill luggage capacity only. There's a clever technique of resting a full-sized rucksack on the rear rack and securing the straps to a dowel attached to the saddle rails that works well for multi-modal touring.
Saddlebags work well with Bromptons. Especially if you're too tall for the standard seatpost.
The bike's handling is somewhat improved with some extra weight on the front.
Cambium black carved or Brompton generic ?
YMMV. I was quite impressed with the Brompton generic, although not to the point of keeping it.
Other Brompton specification tips:
Get the firm suspension block unless you have spine problems or weigh as much as a fairy fart.
Don't get the Shimano dynamo hub; it's crap, and built into a pointlessly heavy wheel. Get the SON lighting or build a decent wheel around the Shutter Precision one for half the price.
Titanium is a rust-avoidance strategy, the weight saving is negligible.