I visit Botswana regularly (I have family there :cool
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My kit is getting dated now and over the years I have used different kit as I have upgraded from film to digital and saved for better lenses (film prints at 4x6 or the slightly larger sizes hid the flaws better!). My latest kit is a 40D with 70-200 f2.8 coupled with a 1.4x TC. I do have a 2x TC, but don't particularly like using it. I am happy enough with this combo, though I would like to add a 120-300 f2.8 at some point in the future.
F4 is fine in terms of giving useable shutter speeds (especially with a modern body), but personally I would rather have the option of f2.8 for the reduced depth of field, as I like to have a nice blurred background.
I think the ideal kit would be to have either a 200-400 and a 70-200 f2.8, or a 500 and a 120-300 f2.8 + 1.4x TC. That would give the best of both worlds for reach and aperture. Take something wider (24 -70 / 24-105 or similar) with you in case you end up really close (I have seen lions next to the road) and for landscapes at dawn and dusk and also for the night sky too (if you are into that kind of thing).
What I have found is that things are either really close, or really far away. The habitat in Chobe and Moremi means that you can't always see that far due to the thickness of the bush.
The sun comes up and goes down very fast there; in the national parks you have to be in camp between dusk and dawn (the guided drives from Maun / Kasane / Nata start at 6am). I find the light gets harsh after around 7:30am and until around 3:30pm in July / August / September time. I have never felt the need for fill flash prior to this, but your preferences may be different to mine.
Just a word of warning, the planes between Jo'burg and whichever airport you are flying into (Maun? Gabs?) are pretty small; my standard size Lowepro full camera backpack with laptop compartment will not fit in the overhead locker, so I hand it over to the attendant as we board the plane.
I appreciate you haven't asked about this, but as a word of warning; don't plan on driving long distances at night. The roads away from the towns are not great (pot holes, poor reflective cats eye markers and lane markings) coupled with the risk of cows / donkeys / goats etc on the road, it makes for a stressful drive with a good chance of an accident. The locals avoid driving long distances in the night if they can help it. In towns it is fine at night, though drink driving is common at weekends.