Check out the new 18-35g nikkor. I tested the 6d with your lenses, and the d600 with that one and the 70-200 f/4 and went with the Nikon set up. I'm mostly a landscape shooter and the d600 has amazing dynamic range. Even after 3 months of shooting non-stop with it I'm still amazed by the raw files when I upload them. The 18-35 has been very well reviewed, is in your price range, and is very sharp, especially stopped down maybe save for the very extreme corners. If you're ok with a cheaper telephoto, the 70-300 may fit, although it isn't near as sharp as the 70-200.