For the rear wheel you could go with something like a
Sturmey 5-speed IGH. It comes in 119 OLD. That would let you get rid of the derailer, have a cleaner appearance, shift gears while stopped. You have the horizontal axle which is perfect for it. Just buy a new rim like a Sun Rhyno Lite. Lace it together. I recently did one. It's not hard.
Is the front dropout really 65mm? I thought Dahon used 74mm. (I haven't found hubs with that OLD. But, thought it would be worth clarifying.).
You indicated you weren't clear what OLD is. It's the distance from the outer face of one locknut to the other. It should be the same as the
dropout space. You can increase or decrease the dropout space a little. For a small triangle like you have, you can't alter it much.