The are a number of different vehicle options available:

2WD or 4WD Fully Equipped Camper (2, 4 or 6 berth campers)

4WD Fully Equipped with Roof Tent such as Toyota Hilux Double Cab

4WD Camping Equipped such as Toyota Hilux, VW Kombi

2WD saloon car such as Hyundai Getz, VW Sharan or Renault Clio

A normal saloon car will be able to handle most of the roads in Namibia and South Africa depending on the time of year you travel and where you want to go.  That said, often travellers select a 4WD vehicle as it does offer a more comfortable ride on the gravel roads and also has a raised driving position which makes game viewing a little easier.  A 4WD will work out more expensive to hire and the fuel consumption will be higher.  We recommend a 4WD vehicle for travel in Kenya or Tanzania, if on a self drive safari.

We have personal experience in using all these types of vehicles and can offer advice on what's good and what's bad about each and their suitability for the adventure you are planning.