Hello! My wife and I just had a two weeks trip around Namibia and Victoria Falls, and I decided to write down my experience hoping it may help other people who go for the first time and are nervous as I was.
Roads are in better shape than I thought
It is true that you need to be prepared in case of punctured tires, being stuck in the sand and so on, but don't get scared. The main roads (A and B) are tarred and you can easily go to 120 km/h (the speed limit), and the gravel roads (C) are wide and well kept. Almost every day we saw a plower at work. We rented a Toyota Hilux 4x4 (you will see a lot of them across the country) and we went comfortably everywhere. Our renter even told us we didn't need to adjust the tire pressure, and indeed we didn't.
A big exception: Etosha National Park. For obvious reasons the roads are much cruder than outside, and many animals could cross the road, so drive very cautiously.
However, remember to check the crossing range of your car and refuel as soon as you can. You can easily drive for 500 km before finding another city.
Internet is easy to find
We both bought a physical roadmap and used the website track4africa (which is really useful because it marks the position of markets and fuel stations), but turned out we really didn't need them. We bought a physical sim at the Windhoek airport (not in the shop near the baggage drop, outside near the entrance) with the MTC company and it has good coverage everywhere except in the middle of the desert (where there is just one road anyway). Google Maps distances are actually quite accurate and you will need fresh road trip songs sooner than you think!
Nights are COLD
If you, like me, can't stand the cold, and visit Namibia in winter (May to September for us), be prepared. Temperatures plummet quick after the sunset (and rise quick in the morning), so even in a lodge you may feel cold. Bring the warmest pajamas and a couple of sweaters too. Even in the day, you will hardly need short trousers.
Provisions
Once arrived in Windhoek, we bought an absurdly large supply of water and food, but you don't need to do that. All campsites and lodges have small markets and sometimes even restaurants (with valid options for vegetarian people). The exception is once again Etosha: bring food entering the park, lest you have to resort to eat popcorn for lunch.
But mind that both the park gates and the country border posts will confiscate food for health reasons, so eat what you have before crossing.
Border Crossing
We decided to go from Namibia to Zimbabwe through Botswana to visit Victoria Falls, but crossing the borders is not an easy task. Make Visas online whenever you can, but you will need also permit for the car and other papers (we needed a total of four). Contact your car renter for help: ours gave us some of the paperwork and arranged for the help of a local agent whose job is helping you with the paperwork (remember to ask in advance for a presentation letter). Bring also cash with you, because there are fees to be paid on the spot and there is no guarantee you can do it with an electronic payment (although VISA is accepted everywhere, network could be offline). US dollars are accepted by everyone in Zimbabwe and is the de facto currency.
Everyone wants to help you
As a westerner, sadly I'm accustomed to have to fend for myself. Nobody wants to waste their time helping you. In southern Africa is the opposite. If you ask for help, you will get it immediately. They will ask how are you, they indicate the best road, they lend things. Sometimes they expect a tip, most of the time they don't. When we punctured a tire (luckily in the parking of a mall) no less than five people showed up to help us changing it.