First off it is a crazy/stupid idea. Theoretically a wonderful adventure, but it is Africa! Likely hood of ending up in prison, kidnapped, dead or at least in the hospital before you complete such a tour is high. You must also know about all the road blocks and cops looking for bribes.... Hell we were stopped every couple of miles in some areas of Zimbabwe. I think you should tour around the areas you know and feel safe, then fly down to RSA and tour that and maybe Namibia. Skip all the discontent in the middle. I never felt in danger in Zimbabwe, just worried about getting ripped off by corrupt cops, however there are good ways to manage that and going up to Vic Falls would be great, I don't know about Botswana. I doubt you are allowed to take a rental car out of the country, might consider buying a vehicle in RSA to tour around and then re-sell it?
You came across as a whack job on here or else you might have even gotten help with a vehicle.
For the discussion, if you are hunting, can't beat a 375 H&H bolt gun for being legal everywhere, ammo availability, and the real ability to be effective on any game animal big or small. If you can handle the recoil of a 300 win mag, you can handle a proper weight 375 H&H. But strictly for protection, you don't need a 375. A 308 can kill a big cat. I'm sure you are in much greater danger from two legged predators. For true protection I would want to be carrying at all times, so that would require a pistol much of the time.... Not practical in Africa and I think the safest thing for such a trip is no firearm as you are too likely to be arrested for having it.
We had a young couple from Argentina with us several months on a training program. They bought a van, several spare tires and gas cans, and toured the US from Chicago to Vegas to Southern CA and then drove home to Argentina. Took about a year as they ran out of money and had to get jobs in Chile to earn money to make it home. Their biggest concern was Northern Mexico and stayed on the US side for the night, crossed at first light will full gas tanks and food and water... Drove as far into Mexico as they could, down the Pacific Coast, without stopping until they got out of the danger zone. Of course they had a common language (Spanish) all the way and were also fluent in English.....
Doing something on the Western Continents might be more sane and just as adventurous.... Personally I want to do Australia as well as NZ and Tasmania. But that is in the distant future.