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 bought aToyota Double Cab Hilux (called a bakkie in South Africa).There are quite a few models of Hilux and our criteria for safari included the following:
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).
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. I don't think engine capacity is an issue. 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.
Up rated suspension for ground clearance/load capacity (two different things). 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.
Spares. Toyota spares are available throughout Africa
Recovery gear. I would suggest a 4x4 training course and an understanding of driving and recovery techniques.
A snorkel for the air intake is supposed to be useful for cooler/less dusty air into the engine. I certainly wouldn't go through water deep enough to need one for that!
Dual battery system if 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.
We have a proper bull bar at the front and solid rear bumper with swing out spare wheel carrier. The spare wheel on my Hilux was slung under the vehicle and the mechanism seized, I had to cut it free and so had to get the new carrier. I've got a winch on the front but never used it in anger (yet...).
Tools and sundries.