If it's about low light, Portra 400 or XP2 400 can be shot at 800, and and I'd say Portra 400 can go beyond that. Also, the theory at least say that you can shoot a range finder at lower speeds than an SLR, due to no mirror slap and also the fact the finder does not black out when fired.

If you can't afford a Leica, then check out Voigtlander Bessa, they are pretty cheap used, and you can get fast lenses easy/cheap too.