Might be riding under a car.

A chainguard should be light and good looking (i.e. none of that Dahon plastic stuff), and not affect the fold. Hm. Maybe a chainring guard would be sufficient.

Whatever the wheel size, might as well make it compatible with what's already out there: if 20" wheels, use Dahon's, Swift's, or Bike Friday's front and rear dropout spacing; makes buying wheelsets much easier. If using ETRTO 349s, use Brompton's spacing, then buy Merc wheelsets.