Hi,
Maybe another solution is to buy both the titanium brommie and merc, swap the bits you want to and ebay the bike you don't want. Even though you might not get the greatest price for it, it might be cheaper than buying bits individually.
I'm not sure the fixed idea would work in the long term - brompton transmissions have a hard life as it is - I used to get through a couple of chains and sprockets every six months.
I read in a few brompton gear-upgrade projects that the rear fork can be spead a few mm, you'll probably get away with it.