Smart move, you really need your own wheels out there because the best spots are miles from anywhere and public transport just won't cut it. Grab a proper 4x4 with decent ground clearance, the gravel roads can get rough and a little hatchback will rattle itself to bits. Check out what is available to rent a car in Namibia here
https://getrentacar.com/en-US/namibia . Also double check the insurance covers gravel road driving, some policies exclude it and that is exactly where you will be spending most of your time.