You will only get a FAA PPL for your RSA Comm. This is what I did a few years ago - got the ATP study material from a Pilot Shop in RSA - studied for 1 month - its very basic compared to RSA Comm and ATP. Go on FAA website and get your RSA license verified by SACAA so you can use your hours as credit. Once your license has been verified apply for TSA clearance for foreign pilot training (you will need to tell them who will do your training and where - flight schools can help you with this). You can do your FAA medical in RSA before leaving. Then go to USA and write your ATP exam (about 2 hours) you get an instant result. You need to do biometrics (can only be done in USA) before getting approval to do any pilot training (this can take a few days) then either do an initial on a type or find a flight school to do an ATP on a piston twin which will be the cheapest.
Good Luck