I am person who bought a brompton, a merc and was disatifed with both. I felt I had believed a little too much brompton hype and should have taken more care before comitting myself. I bought the merc to upgrade whilst running a standard brompton for the commute.
I sold both and bought two mezzos which I test rode extensively before buying, so I realised about under gearing. I really suggest you test ride any folder before purchase as they are so different from each other.
However if you feel brommie package right for you after test riding rivals also then go for it. They also had resale value well, so mistakes are not too bad anyway.
Since selling brommie I have bought another very cheap and sloved many of the issues I do not like about the bike with upgrades and mods.