Hi Simon,
No,you would not have to buy B&O speaker cable, any good quality speaker cable would do.Richer Sounds will sort you out with that no problem. You will need 2 pin din connectors into the 9300 , and standard RCA connecting plugs for the Missions ( or just about any other make of speaker). If possible get 2 pin din connectors with the screw terminals rather than soldered - much easier to use!You can indeed connect 2 pairs of passive speakers to the 9300 but please note pressing mute will only silence 1 set.
It is not my intention to steer you away from the RL speakers, I am sure they would be excellent, provided they are in good condition. They certainly are totally in keeping with the visual attraction of the system. The 9000 series are 1 of B&O's classic systems and always popular on this site. The mission speakers visually are not in the same league but sound wise my 700's are a very balanced speaker and certainly worth a listen.
I'm guessing you may be on a budget and that is ruling out buying any active speakers (BL 6000, BL 8000 etc.) but don't forget the 9300 also has power link connections for B&O active speakers.
Whatever you decide I hope you enjoy your system. I'm sure you won't be disappointed.
Graeme.