Phil,
I'm not sure I would buy an 851 new. A used 850 will sound great. Other things that pop into my head are mainly circling around overkill. If you are doing 2 channel through a Cambridge, an oppo 103 will do just fine for theater and save you a bundle. That's savings #1. Also, if you have a Cambridge and Oppo, there's really not much that you need out of the Marantz, is there? Just 5 channel volume and maybe different modes. The Emotiva mc200 or outlaw will do the same very nicely and do a darn good job of it. Savings #2.
I know that my Emotiva passes a pure audio signal through very clearly without any coloration. (the marantz 7007 may do this too?) So, I'd say both the Cambridge and/or Oppo could run audio direct mode. You won't need a 2 channel pre-amp. Savings #3.
Now that you have saved money, blow the whole load on a really killer multichannel amp. The Emotiva amps are nice and a great value but not in the class of Mac, Classe and some others in the $5k range.
So, where would I start if I were you? Buy a nice 2 channel CD or DAC-Cambridge is awesome. Try this on CD direct on the Marantz. After that, try borrowing pieces. I think you might be surprised where the upgrades will come from after that.
My theater: Emotiva UMC200, Oppo 105, Dishnetwork, Mac 5 channel amp, coincident speakers all around, velodyne sub and a sota table with sumo phono pre for analogue. I also run my laptop with foobar through a music fidelity for streaming. Don't forget nordost cabling and PS Audio conditioners.
