The previous posts have been very interesting to read, and overall I would agree with most.
Living in Durban, and traveling to KNP regular, I can give you advice on the exit strategy, in particular from the Park. Also, due to your limited tme and access to be taken into account.
I would advise to not
travel to Orpen gate, from Jhb OTI, unless you are staying overnight somewhere en route. While the route on the map does not look long, actual travel time is always worse than expected...
And if you doing a lot of traveling you will certainly be tired and not enjoy your time.
If I can suggest, based purely on ease off access from Jhb and less strain on yourself, but with still excellent game viewing opportunities:
1st Night - Malelane or Berg n Dal (Malelane Gate) or Pretoriuskop (Numbi Gate) or Skukuza (Phabeni Gate)
2nd Night - Skukuza
3rd Night - Satara
4th Night - Satara or Lower Sabie
5th Night - Lower Sabie or Crocodile Bridge
All of the above offer Evening Drives and Morning Walks, and withthe exception of Malelane (which I highly recommend though), there is a supply store and fuel available.
By exiting in the south and using Crocodile Bridge as the exit point, you will have a better position to drive through Swaziland to get to KZN.. as through Swaziland is recommended route from KNP.
Remember to have your Driver License, Vehicle documents from Car Hire company to allow travel through Swaziland.. and of course your Passport...
Hope this helps
Good luck with your final decision.