Hi
My mate has just returned from SA and he broght a long-wheeled base landrover vehicle in Cape Town to tour round southern Africa. He said he didn't have any problems finding one - he got his from an advert in a local paper.
I guess Jo-burg will be much the same. He paid 2,000 pounds sterling and traveled all the way round southern africa without any problems.
He did recommend that you make sure it has a RWC Certificate (Road Worthy Cert) and that you take out insurance - which he was able to buy on a monthly basis quite cheaply.
Hope this helps.