not very i rented a car at the airport and it is fine - you do need to take your passport & liscense - and you have to buy insurance
there is really no where to rent other then the airport so ask the different places there what they cost i rented from durban airport - the people were very nice and i had a brand new car!! i got a toyota runX it was a nice car compact hatchback - TONS of room for my stuff & people!!
some tips - DO NOT STOP FOR ANY REASON ON A HIGHWAY!!!!! always carry change to pay the car guards (they have people who walk around parking lots and watch cars) usualy 2 rand pieces or 5 rand are good.
you drive on the left - like england.
the biggest problem it 99.9% of all cars are stick shift - so i hope you can drive a manual!!! if not you will pay A LOOOTTT more for a automatic!!
i loved south africa i have been there a few times even lived there for a while!!