Let me answer that last one: on speed and low light shooting. With the K-7, a fast prime lens will allow you to shoot at much lower ISO and you will get great shots.
I shoot often with the Voigtlander Nokton 58mm f1.4 even in low light, and I love this fast lens. In addition to the superb IQ, the fast lens coupled with the K-7 works very well and I am sure that oui 50mm f1.7 will give you great shots on the K-7.
Similarly I get great shots with the FA31mm f1.8, but the lens is as fast as the Nokton 58mm f1.4.
Hope that the comment help.