First of all, I don't think it's wise to go to Africa.
I think, an entry level mountain bike from one of the major manufacturers is the way to go.
They are rugged enough right out of the box to tour on.
I like the Specialized Hardrock, $390. There is a cheaper Hardrock XC for $330 but it's not much cheaper. The money saved is not worth it.
The frame is beautiful, well made, and incredibly strong. It will endure mountain biking so it will endure touring.
The Hardrock has 36 spoke wheels which is unusual. A 36 spoke 26" wheel is mighty strong.
It also has as low gearing as you can get on a production bike. 42/32/22 chainrings and 11-32 8 speed cassette.
Buying a used bike can end up costing you more than a new bike. You're a novice to bicycles and you wouldn't know what to look out for. When you buy new everything is new. In a bike shop you can try different sizes back to back and get the size that feels best to you. You have a warranty and the bike shop will assist you with no charge during the break in period.
Normally I wouldn't recommend a bike with a front suspension fork. It adds weight and complication and really isn't necessary. But it's hard to find a mountain bike without one.
Fortunately, there are front racks designed to work with them. The chainstays on a mountain bike are short so you'll need to find a long rear rack to avoid heel contact. An Xtracycle might be a good alternative.
It would cost a little more than racks and panniers but they say it softens the ride somewhat.
You would have no trouble carrying all the gear you need on an Xtracycle. People have done around the world tours with Xtracycles.