You mentioned you were interested in low-light photography. In that case, the 5D2 will not fit your needs, but the 6D would beautifully (as of course would the 5D3). I love my 6D for it's low light capabilities and while the 5DIII does more, you can put the money you save on buying a 6D rather than the 5D3 on or toward another lens. I guess it all comes down to whether you'd really get the use out of the extra features of the 5D3 more than you'd get from another lens.
Can you explain what you mean by low-light capabilities? And why would the 5DII not fit his needs for that? I never had any issues with my 5DII in low light settings. Quite the opposite actually. I'm not sure how the 6D is even better with that. But even if that is the case I doubt that that has more practical implications than the 6D's lower x-sync speed, missing 1/8000 and the small plasticky form factor.
The 5DII is not only a bargain at this point - at least for my needs I'd still consider it the better camera. The MarkIII of course solves all those issues pretty much but still comes in with a much higher price tag and I'd only shell that out if I'd really need the additional features and upgrades over the MarkII. I'm in no rush with that personally.