It would be smart to start with the Hanoverian Verband in Germany if that’s where you believe she came from. Every horse that is branded (legally) should be in their register somewhere. You may need to provide photos and copies of any paperwork that you have on file.
Also, if she was imported to South Africa, clearly she got there somehow. There should be records of her transport and quarantine. My understanding is that horses imported to South Africa are quarantined at Kenilworth Station.
It will definitely take some digging, but those are good places to start.