The road from Windoek airport to Swakopmund is very good, if not the most scenic. from what I remember, we did it 4 hours or so.
The road from Swak to Sesreim is a different matter. It is very good (if a little busy by Namib standards)to Walvis Bay, but once you are in the Namib-Naukluft National Park which begins I think about 20-30Km outside Walvis Bay, on the way to Sesriem. The road becomes unsealed for the majority of the journey.
It also becomes very windy and slow as you go over/through the mountains, but this is only for an hour or so.
But dont let this put you off, it is a spectacular drive, and can be achieved in a two wheel quite easily, as long as usual precautions are taken. Also - have a full tank of petrol entering. Always good advise anywhere in Namibia.
If you are going to hire a car in one airport and drop if off in another, check if there is a charge, as I remember them being quite steep (due to return distances)in some cases.
Hope this is of some help