Just returned from a 3,5 week selfdrive round trip in Namibia, Zambia and Botswana. We rented a 4x4 pick-up with rooftop tent and camping equipment from Advanced Car Hire in Windhoek, after selecting a few reputable companies according to TripAdvisor and Thorntree reviews.
Pleasant and swift email contact with Carla from Advanced to organize things. Pick-up from the airport free of charge, proper check-up and instructions on the car, tent and equipment and on our way in no-time in a couple months old Hi-Lux with 3 spare wheels/tires, fridge, maps of Namibia and an owners letter for permission to take the car out of the country, provider free of charge.
We drove over 6600 km's, a considerable part offroad, without any problems. Turning in the car was hassle-free, as was the ensuing free of charge return to the airport. Best of all: most competitive price we could find in Windhoek for this package and setup.