I did a Safari in Tanzania Dec last year.. My recommendations are 2 bodies, one crop with atleast 400mm and one FF/crop with normal to wide.. The animals are amazing, and the 400mm really are needed, I often felt like that wasn't enough on crop. But not to be frogotten the landscape sure is stunning, you will want to get some shots of it without changeing lenses. I personally like to take more casual shots of my travelling companions etc, and here the normal/wide will come to great use.
And yes, the dust is sometimes unbearable, atleast. On my 550D I'm pretty sure it got in even if I only changed lenses in the tents, never in the field or in the car. However, I guess it would have been worse if I did change all the time. If it gets really windy I would recommend a UV/Skylight filter atleast to protect your lens, and if you really want to baby it get som covering for the lens body too.
Now don't forget that this will be an amazing experience in every way. Enjoy and I hope you get alot of good photos!