Have owned roadbikes(Fuji, BMS, Scott, etc.), a raw lacquer Brompton and a titanium/super light Brompton.

Some of your questions will be answered by a test ride or two. 6 vs. 2 speed, lifting both standard & ti models, etc.

Since you have a multi geared bike; I'd suggest to go with a 6 speed. Around US$200 difference when buying new?
If you switch to 2 speed; simple enough - just buy a 2 speed wheel. Going from 2 to 6 will cost a lot more; wheel,
shifter, cable, der., etc. + labor. my first B was a 2012 raw lacquer 6 speed. Got it while on vacation:
Philly by 1nterceptor, on Flickr

I liked it so much; I sold it after 2 months. I figured the only thing better than a Brompton was a lighter one.
Got a 2013 titanium/superlight 6 speed. Can hold it's own. Been known to keep up and pass some roadies:
100 Miles(160 Km.) on a Brompton by 1nterceptor, on Flickr