Dahon's archive says their 2005 & 2006 Vitesse D3 was fitted with a SRAM T3 internal gear hub. Since you say the one in question has "some sort of revolving SA hub-gear contraption on the back" then it's been modified with who knows what and its almost impossible to give meaningful advice concerning final gear adjustment and getting the cycle back in service.
Shimano, SRAM and Sturmey-Archer all have step-by-step guides for wheel fitting for all their IGH models on their web sites if you can ever figure out with what it is you're dealing.
tcs