I have just been back from 2 months nomading in Mozambique, Tanzania, Zambia, Zimbabwe and Botswana. And I have resided 1.5 years in South Africa so I can share you some tips.
As you may expect it, the internet is not great there. In Mozambique, you can buy a SIM card from Vodacom and get 2GB for no more than USD10, cheap, and the connection in 3G is good. I had more ease working on 3G than on most Wi-Fi hotspots.
You can do the same in Tanzania with Airtel, but the quality of the network is quite poor.
I don’t know about the 3G in Zambia, Zimbabwe and Botswana, but I found it hard to find Wi-Fi in most hotels.
In South Africa the connection is good and most places have Wi-Fi (at least compared to the countries above), the 3G is good but expensive.
I have been 2 weeks in Zanzibar and let me save you some troubles if you decide to travel around the island: withdraw as much money as you need in Stone Town, the capital city, because then there is no ATM on the whole island!
This region is fantastic, relatively unexplored as compared to many typical nomad Asian cities. The 2 main downsides of traveling there is the difficulty to find proper internet connection, and the relatively high prices of everything. Mozambique and Zanzibar are relatively expensive (especially the North of Mozambique, region of Pemba). Zambia, Zimbabwe and Botswana are surprisingly expensive. South Africa is cheaper and probably more suited to nomading.
Don’t hesitate if you need more info, I’ll be happy to help.
Cheers, enjoy your nomading in this awesome part of the world!