Wow! love the illustrated custom Brompton…always
1st I’d recommend getting a test ride , to make sure it suits, and decide on gearing options based on that.
I run a 2speed for lightness, and carrying, (some titanium helps too ;-)…)
Schwable tyres are really reliable, and available in a range of styles depending on terrain–speediness.
Something I find really helpful is adding some micro barrends , like X-Lite ‘knobbies’ from a long time ago, and being a mtb ride I squeezed some cut down X-Lite XC risers in as well — however handlebars is an area that Brompton has really progressed, and there are numerous styles availible now.
I used mine a lot, commuting, ragging around the Glastonbury Festival site (when not muddy)…even offroad following friends around the LeighWoods tracks. Some fairly long rtouring type rides too, made comfortable because I was running my preferred mtb saddle / grips / and nice Shimano XT pedals. Overall lots of fun, to be had and very useful too. Happy Bromptoneering!