SD, I'm envious of your planned trip, and wish I was back there now myself. There is some info in another thread that might help you too. Although I didn't go up north, I did a self-drive and camping trip in the southern deserts after my hunt in '08. You might consider hitting the Waterberg Plateau for a hike / camp for a day or two. You can then either go to the eastern entrances to Etosha northeast from Otjiwarongo, or northwest through Outjo to the south entrances. It depends on what type of accommodations you want. If you have the time & or inclination, you could even head southwest after that through Omaruru and on to Swakopmund. I did spend a couple nights there, and enjoyed the place. If you do go there, the Brauhaus was disappointing, but seafood and wines at The Tug were SUPERB !!! There is a quality photo shop called Behrens on Tobias Hainyeko Street just off the plaza that I used to make prints and burn to a CD. It was nice to look at pics on the long place ride home. Lunch at the Lightkeeper's Cottage was pretty good as well. Lots of things to see and do within walking distance. It's an easy half-day's drive from there to Windhoek on the way out. Sossusvlei is stunning and you can see most of it in a day, but it's a looooong loop on rough roads coming from the north. Fish River Canyon is simply too far south for the time you have allowed.
IIRC, I used Advanced Car Hire out of Windhoek to rent a rooftop camper. There are advantages and disadvantages. If you're going to stay at guest farms and drive on tarred roads, a 4x4 or even truck is not necessary. You can rent good cars for reasonable rates and get there in more comfort. If you do choose to camp, the rental camping gear is usually in decent shape. The rooftop campers allow for a nice breeze in the night, plus gets you off the ground away from the scorpions, sand spiders, and some snakes (cobras, puff adders, etc) but mambas and boomslang can climb readily. VERY small chance of any of that happening though. IMPORTANT: if you do camp, plan ahead with you PH and have meat from your kills packed for your camping trip. There are always braais about, and kudu, gemsbok, hartebeeste (or ELAND!) make for fantastic dining al fresco. I was most pleased with the arrangement. Good trading material too when you ask for directions or advice from local folks.
I would HIGHLY recommend the following book for detailed resources, maps, reviews and prices of different facilities:
"Namibia, The Bradt Travel Guide" by Chris McIntyre,
Bradt travel guides ...take the road less travelled, or readily available at
Amazon.com: Online Shopping for Electronics, Apparel, Computers, Books, DVDs & more. It has info on everything from hotels to campgrounds to campsites.
If you need further, send a PM and I'd be glad to help.
Waidmansheil !