I definitely agree about dust, that was the first thing I thought of when I read your post. I grew up in Zimbabwe and Botswana and dust was a major issue. I would seriously think about bagging my equipment, particularly in and around Botswana.
It's been a very long time since I was over, but I don't recall anything about xray issues, I think you'd want to buy your film in Joburg or wherever you fly in, and after that you will find that the checks at border crossings are very spartan. Expect no fresh photo supplies whatsoever, and then you might be pleasantly surprised in some of the larger cities.
Regarding lenses, I don't think you can ever have a lens long enough in Africa. The distance scales are quite a thing. Even more so than the American west- you think you want a wide lens to take it all in, but it just doesn't work. If you can make it work then more power to you... most people cannot.
If it were me, I'd go with 35mm gear and I'd honestly try to take a 400mm lens and a 70-200 zoom. It'd be good to have a normal kit for street photography, which can be very, very interesting all over southern Africa, but if you want to do wildlife and landscape, you can forget wide. Oh and take two film bodies for sure.
Do check whether you will be there during the rainy season as that presents a number of its own advantages and disadvantages depending on what you're after. Needless to say, the landscape is very dynamic around the rainy season. Think you've seen rain before? Be prepared!
Wish I could go back <sniff>