the "B" usually means black veneer. The Polk CS1 is what polk recommends for the Monitor 50 and Monitor 60 towers. The CS2 is recommended for the larger Monitor 70 towers. However, a lot of forum members have worked with both and like the CS2 better, regardless of which Monitors are used for the L & R. For your large room the CS2 would probably work better, but it is a large speaker enclosure - so make sure it will fit your setup.
as surrounds, the Monitor 30's would be better for the "music" part of your listening.
Yes, Monoprice is the place to get wiring and interconnect cables. I have used quite a bit of their stuff, both for work and family. Excellent quality and the prices are outstanding.