In a hostile environment, you will want weather proof equipment.
1 series cameras. Weather sealed lenses ! A backup camera because they do fail under difficult conditions. I'd not take a studio camera like a 5DS or R. They may survive, but there are good choices that are more rugged.
You have two good lenses that are weather sealed, you are missing a 1.4X extender.
I'd try to find a good 1 series camera, even if its used. Its far less likely to fail in a cold wet climate.. A 6D is just not a rough weather camera, the 7D MK II should survive.
So, things to consider: Sell the 6D and get a used 1D MK IV or 1D X, or a 1DS MK III plus 1.4X TC, and, perhaps a 16-35mm f/4L instead of the 17-40mm L.
Get protective filters and covers for your gear and a water proof bag. Make sure you have a suitable circular polarizing filter that will work.
Thank you for your opinions.
I am, indeed, missing a 1.4x extender. It is already on the upgrade list for this year. Do you think the 1.4x III offers any improvement over 1.4x II when paired with the 70-200mm IS II (or even the 100-400 II in case I buy this lens one day)?
A 1D X is out of my budget, unless its price falls down even more this year. A 1Ds mark III costs US$ 2K - 2,5K in Brazil and that might be an option. Although I have already bought some used lenses, I am not very fond of buying used cameras, but I will consider this option.
All my lenses have protective B+W UV filters, and I have a Hoya circular polarizer which fits my largest lens and step-up rings for the other lenses.
I am worried about weatherproofing my equipment. I am thinking about buying rain covers for the cameras and lenses to used if it snows. I am in doubt between Manfrotto E-702 PL Elements Cover and Acqua Tech All Weather Shield Primary. Do you know any of these covers?
I may not have phrased my original doubt very well in my first post: Which of these lenses offers more improvement over its predecessor: 16-35 f/4L IS over 17-40 f/4L, 24-70 II vs 24-70 I, 100-400 II vs 70-200 IS II with 2x III?