ECO D7 is cheapest and lower gear range.
Speed D7 is also inexpensive, and less low gear range.
Vitesse D7HG is aluminum + internal gears
Speed P8 has better components
Mu P8 is aluminum + better components
Curve D3 is nice with 16" wheels, and internal gear but only 3 gears.
If you are set on Dahon, I would recommend Speed D7, but many other opinions out there, and it does depend on you as well.
NB need to make sure you want, or whether you need to get rack, whether the provided rack is appropriate, panniers, lock, outfits, accessories etc. And 2nd hand is an option.
Dahon-specific forum at
www.dahon.com/forum