That series has a very distinctive tweeter sound and you should look for the same series center, the HTM2, HTM3 or HTM4 would work. If you don't play too loud you could always use a N805. They just changed 800 speakers again and some of them are now Diamonds.
B&W always had an odd mix of surrounds, some are/where pseudo dipole and some monopoles. If would be up to your layout as to what type of speaker to use. If you aren't doing strict music surround then even the 600 series does fine. I've also had good luck with many other speakers brands that have a similar timbre as the fronts at least using a metal dome tweeter toward brightish but not bright enough to call too much attention to itself.
Subs, use a good one but it/they don't need to be B&W. Chose the sub (s) solely on performance. JL, B&W, Paradigm, Velodyne and many others. Your best bet here is to leave a little room for an equalizer, unless built-in somewhere, to smooth out room/placement issues.