I think this would be a great b-roll camera to the 900- though of course you’ll definitely see a difference in quality- vis a vis- difference in CCD size, lenses, compression schemes.
You won’t need the Kona. FCP 4.5 was called FCP HD when it first arrived because it stood for DVCPROHD. It is already optimized for capture and playback of DVCPROHD via Firewire. 800 drives are solid for this purpose. I would suggest upgrading to FCP 5 simply because it’s even faster and more solid than 4.5 plus many new upgrades.
5.1 is automatically downmixed to stereo on any non 5.1 system so it’s pretty much impossible to keep anything in the surrounds discrete. I’d advise creating an alternate audio track instead with the SMPTE time code in it- that way you can switch back and both.