Steve
I went on the SA/Zambia trip last November/December (brilliant!), took 5000R with me (plus $800) and withdrew 6500R, but I was very extravagant. Budget for 5000R while there if you plan to go on the excursions. There are plenty of opportunities to use ATMs when travelling, as long as you do it with someone else (although I did it on my own a couple of times in daylight where there were plenty of people about). The guides let us know whenever there was somewhere we could get cash. I never had any problem with reliability.
Have a great time, you won't regret it.
Mike