I have a 20"/406 wheel bifold, 4130 chrome moly steel frame, usually better for fatigue life, but aluminum frames are better than they used to be, gusseting has been added.
Mine was 7 speed, not low enough for hills. I added 2X crank, but this took some engineering as the frame has no front derailleur bracket, and the combination of fat seatpost/seat-tube, plus clamp on FD adaptor thickness, made positioning the FD far enough inboard difficult. Careful grinding of the FD inner cage lip where it interfered, solved. But a folder frame with a road-style FD bracket ("braze-on", actually welded) makes things easier. These days, most folks just mount a wide-ratio cassette for lower gearing, I recommend minimum 400% range. My 2X gearing is 50/34 11-30, 21-85 gear inches. When I replace the cassette I will go to a 34T low cog. The rear derailleur is a "mid" (GS) length, which has more than enough capacity, but more ground clearance than a long cage.
A rear rack to hold panniers is easy, provided it mounts far enough back for heel clearance when pedaling. Front panniers mount on dedicated 20" racks, though I used a cheap "brake post" rack and added wood sticks to keep pannier out of spokes and hung panniers. Cargo area above the front rack deck is fantastic for large parcels.
Here are helpful pics:
Loaded touring, note rear rack position, I needed to buy extra long rack stays and adapt to rack:
Panniers off and folded on train luggage shelf; If space was tight, it could go vertical in a bag or trash bag over top, to keep oily chain off others' luggage; I plan to go back to waxed chain, cleaner:
This is a Kevlar cable (stiff as steel), tight, that I added to mimic the effect of a steel cable there on Dahon bikes with Deltech; It transforms bending stress at folding hinge to compressive shear, this improves frame strength and reduces torsional flex of frame under climbing. This helps to bring the frame closer to a fully triangulated frame like a Bike Friday Diamond Llama or All-Packa, but at much lower cost. (before tying off line ends):
