Practice shooting standing off of sticks. Be comfortable out to 200 yards off of them. Almost all shooting is off sticks. Noted above, but important enough to state again. You pay the fee whether you wound or kill, so make damn sure you kill.
Find out what rifles they have. Taking one is not hard if you don't like their options. Sometimes they have nice loaners, sometimes, not so much.
Depending on time of year and area, mornings can be 30 to 40F. Sitting on the back of a truck in the open air, you will get cold. By 9am, you will be in shirt sleeves. I take a windproof jacket, heavy wool sweater, gloves and beanie for the mornings. The sweater is good in the evenings as well.
Pack one change of clothes in your carry on minus pants. Wear clothes on the plane that you can hunt in including shoes. Pack your binocs and camera in the carry on. Put NOTHING of value in your checked bag.
Mooseman has covered most of the rest. Get the price list. Find out if any animals are on a sliding scale for size and how/when is price determined.
Ask if you are hunting one farm or several. Sometimes there is quite a drive to get from one to another. All animals aren't always on one farm, so you go to the ones with the animals you are after that day.
Expect to be annoyed by folks at the Joberg airport and all of them have their hand out. Don't let anyone touch you stuff, and don't be afraid to be rude about it. I hate that airport for that reason.
Jeremy