The hardest part is deciding which particular vehicle to buy. There are quite a few very capable models on the market. Depends on how much you want to spend and the time and effort required tracking the right motor down.
We've been very happy with our Toyota Hilux (called a bakkie in South Africa). When travelling in Southern Africa you'll see that the vast majority of vehicles are Toyotas. Once you've got the vehicle you also need to consider what extras you might want.
Diesel. In Malawi, on a previous trip, petrol was scarce but we didn't have trouble filling with diesel. Farms rely on diesel and if you're near a farm the chances are they will let you have a few litres (at a price). Carry a spare fuel filter. Unclean fuel can clog a filter (not good out in the bush) though it's never happened to us - yet!
Long range fuel capacity, usually in the form of an auxiliary tank offers piece of mind and is better than jerry cans. About 160 litres total would be sufficient.
Engine capacity. 2.5l with low range and diff lock is adequate for serious off road work. Pole pole (slowly, slowly) and you'll get there and save on fuel. For towing a trailer or caravan a bigger engine capacity would be better.
Good tyres. I made the mistake of considering price too much and paid in punctures. The BF Goodrich we now have have turned out cheaper/kilometre. Try to get a second spare wheel or at least a tyre and know how to repair/change a tyre. Carry puncture repair/tyre changing kit, a compressor and pressure gauge. The chances are you can find someone willing to do the labour if you have the kit! (even out in the bush).
Up rated suspension for ground clearance/load capacity (two different things) is useful. Depending on the set up you will need to be able to jack up the wheels for changing. I use 2 hydraulic bottle jacks. One to raise the body and one then to raise the wheel from the ground. Two (folding) axle stands are used before I will venture under the vehicle. Don't rely on the jack supplied with the vehicle. Carry some plywood/timber to spread load and provide flat surface for the jacks/stands.
Spares. Toyota spares are available throughout Africa I carry: filters and belts, oil and fluids, spare bulbs & fuses and the usual bits and bobs.
Recovery gear. I would suggest a 4x4 training course and an understanding of driving and recovery techniques. It's no good having the recovery gear if you don't know how to use it!
A snorkel for the air intake is useful for cooler/less dusty air into the engine. I certainly wouldn't go through water deep enough to need one for that!
A dual battery system is essential for running a fridge/freezer.
Roof rack. When venturing into the 'bush' I put a spare tyre on the roof rack. Before we had a trailer/caravan I kept a couple of plastic crates or a box on the roof rack are for carrying charcoal and firewood and other stuff you wouldn't want in the vehicle.
Bull bars. We have a proper bull bar at the front and solid rear bumper with swing out spare wheel carriers. The spare wheel on my Hilux was slung under the vehicle and the mechanism seized, I had to cut it free so feel much happier with a carrier on the back of the vehicle.
Winch. We had a winch on our previous vehicles but, ay yet, haven't had one fitted to our current one.
Tools and sundries. Depends on your own preferences and abilities.