Hi Ben, I'm just back from 4 weeks in Tanzania, so my tuppence worth.
The 5DMKIII will do you just fine, I use 1DX with 5DMKIII as my back up, both these Bodies are amazing and I've not had any issues with the 5DMKIII weather sealing in either Dust/heat in Africa, or Snow/Cold in December in Japan.
The area you may suffer a little, will be Length, Lens Range, not anything else.
Your view to add a crop Body to your FF 5DMKIII is the right way to go, that 300f/4 with 1.4x plus 1.6x on a crop body will likely give you all the Range you will need, I rarely find I need more than 600, unless your a Birder, then out to 1200 still may not be enough.
Looking at your options, I like the New 70D to go with your current Kit, you can always off load it when the Mythical 7DMKII eventuates and in the meantime you have a pretty good set up.
I have the 1DMKIV and I use it mostly for Macro underwater Photography, took it on my recent Tanzania trip to try with my New 200-400f/4, didn't like the Image Quality against either the 1Dx or the 5DMKIII.