Hi Patrick!
Not selling mine, but I highly recommend a Brompton - it's just a very practical bike to have.
"Can you fixify Brompton drivetrains?" I'm sure someone has figured out how to do so - the brompton hacks (facebook) community is full of ideas. The main issues will be the fact that the fold happens behind the bottom bracket, so there is a chain tensioner to take up slack when folded (see photo 1 & 2). The second issue is the more/less vertical dropouts (photo 3).
If you can figure out a (safe) workaround for those issues, then I guess it's possible to fixify. But yeah, maybe not worth the hassle.
Another option is an older Dahon Boardwalk, much cheaper, still fairly classy looking, and you could very easily fixify (folds in half, rather than thirds like a brompton - the fold doesn't affect the drivetrain).
cheers,
John