I'm just back from Namibia; had a great time there! To echo Palal, if you intend on driving in Namibia you should rent a high-clearance vehicle that's either 4x4 or AWD-capable. Most people drive a Toyota Hilux (pickup truck). This is an ideal vehicle for getting around Namibia's numerous gravel "C" roads. Renting a normal car is a great way to get into trouble that you can avoid by simply renting a Hilux instead.
With respect to visas, as a Canadian I didn't need one. They simply stamped the passport as I entered Namibia and again when I left. The process of getting in was to fill out a one-page form with the usual questions - name, nationality, DOB, passport #, arriving flight # etc. You're requested to list where you're staying in Namibia. I just put "Windhoek" as that was an accurate response for my first night. You're also asked how much $$$ you'll be spending while in country. I had absolutely no idea so I just put down some nominal amount; I think I may have put $10,000 Namibian or something like that. Once you've filled your form, you line up and wait your turn for an officer to do whatever it is they do once they take the form from you. I was amongst the first off the plane and the whole process took about 10 min. I wasn't asked any questions about why I had filled out the form the way I did.
When you leave Namibia, you need to fill out the exact same form. So for address in Namibia, I just put down "departing today" and I didn't put anything for the how much money are you spending. Again, no questions asked once I handed in the form.
On the whole, it's adequately efficient. It helped that the form was passed out on the plane so by the time we landed I had it all filled out and ready to go.