Since you got the bucks go with an internally geared hub, like on Dahon Mu XL. Mu SL has a derailer that will require more maintenance. I think you will be happy with 20" wheels on a train. On a crowded bus you might want 16" wheel like the Dahon Curves.
FWIW:
Dahon has a female frame with similar parts.
Worth considering is Strida which might work for your commute.