After you bought a car
Registration
You need to be in SA with a local address to get the car registered in your name, so if you are buying an out-of-state car think about how far you will have to drive to get the papers change, and whether you are going that way anyway.
Insurance
Make sure you organise insurance before you drive the car away.
Servicing
Check whether the car you are looking at has been recalled as part of a safety campaign and, if it has, ensure the repairs have been made.