Getting to Sesreim
From any direction to get to the Namib Deserts entrance at Sesriem you will have to travel some distance on a gravel road. The good news is you do not need a 4 wheel drive vehicle to visit, you can take a normal sedan vehicle, I have driven the route in a vw polo with no issues, the gravel roads can be a bit bumpy and corrugated in places but with cautious driving, limiting your speed and taking your time it is possible to manage the journey easily.

Driving into the Desert
From the second gate at Sesriem there is a tar road in reasonable condition all the way to the 2 wheel drive parking area (a flat hard surfaced part of the desert). From there you have a choice to walk into the desert or pay for a Namibia Wildlife Resort 4 wheel drive shuttle that will take you through to the 4wd parking area from there you can walk into Sossusvlei and pick a dune to climb.

If you pay for the shuttle its a return fare and they go every 30 minutes from sunrise to sunset.

Sports cars
Vehicles with low slung suspension or sports skirts will struggle on all Namibias gravel roads as often there are sharp dips in random places, if you have to slow down to crawling pace to creep over a speed bump in your vehicle its probably not going to remain intact in Namibia.
This is not an issue with rental vehicles but more for people driving up from South Africa in thier own vehicle.