Namibia accepts most driving licenses throughout the world.  You would only need an international driver’s license if your current license does not adhere to the following:

• Licence must have a picture of the holder;
• It must state “Driver/s License” or “Driving License” in English;
• There must be an expiry date and this must be valid for the duration of your travels

However, to obtain an International License is always recommended where possible. It's important to keep in mind that you check any requirements for any other countries you may visit cross border with the vehicle.