Hi,
Some view points.
I would say that a Canon 5DIII is perfectly good enough for landscape. I was shooting along with Hans Kruse in the Dolomites a month ago, Hans was shooting both Canon 5DIII and Nikon D800 and he said that both are OK. With Canon you need to expose ETTR with Nikon less so, because of the better DR. I have been quite happy with the few Canon raw images I have seen.
I am pretty sure that Nikon has the better sensor right now, but one can speculate that Canon will catch up.
Regarding the Sony A7r, I am pretty sure that the shutter vibration is there. I have seen it in both test chart shots by Lloyd Chambers and MTF data from Jim Kasson. The Sony A7 does not have the same problem. Sony is pumping out new models all the time, and I am pretty confident they will have a replacement for the A7r. I also feel that the A7r was a bit pressed rushed into market.
Sony A7 has the advantage that you can mount almost any lens on it, but far from all lenses will actually work.
I guess you could go with any lens. For me (not a Canon shooter) the obvious choice would be the 16-35/4, or one of the T&S lenses.
I can also mention that I don't feel there is something like a landscape lens. I shoot landscape with every lens I own. Fisheye, ultrawide, normal zoom, short tele, long tele. The ultra wides are probably the ones I use least. Bernard forgot about telling you to stitch, so I add that stitching is a better way to widen view than using wider lens and crop.
Best regards
Erik