If you mean a codec, you are correct - Canon has no 64 bit codecs for the 5D3, they also have none for the 5Dc, 5D2 or any other camera. Neither Canon and Nikon have created 64-bit codecs. Microsoft has some or you can do like I did and pay $15 a 3rd party set of codecs - they work great.
As for camera choice - I personally would not buy a 5Dc, I just don't like buying used bodies - I prefer to get a warranty. I also don't care much for the LCD and no LV. Plus you already have a great FF lens lineup. I think either a 6D or 5DMKII would work best. I went for a 5DMKII, last week, got a great deal $1,439 - new, USA, very good dealer. Money was a major factor for me, but I also like the control layout. I will be using the 5DMKII with my 7D at events. I personally prefer a similar control layout and size of the 5DII.
I do think the 6D looks great and if you have the money it might be worth the extra money. I doubt I would have used the wifi, but the GPS would be nice. I know others really like the wifi so the extra 6D features like wifi, GPS, AF etc.. may make it a better choice for you.