The South African DCA/CAA has always been rather neurotic concerning medicals. At one time they were done by the military at MMI. The situation is a little better now.
I am sure that you will be required to undergo a full South African medical which includes a stress ECG. You may also have to put up with the fickle finger prostate check, although I believe that this is not now an absolute requirement. The last medical I did down there the doctor said that he offered this as a service. I declined, having recently had a PSA check.
If you are flying down to Johannesburg, give yourself a day or two after arrival to acclimatise to the altitude and to allow your blood pressure and urine to stabilise. For example, sitting cramped up in an aircraft seat can throw out a temporary blood trace and dehydration can lead to a protein indication, neither of which you want on an initial medical.
I think that on the SA CAA website you will find a list of medical practitioners who can undertake the initial. The system is not as user friendly by any means as is the CAA medical section in the UK.
