There's a bus that runs, I believe, 3 times daily from the airport right into the Sun City complex. By South African standards it is hopelessly overpriced, at close to $30 each way, but obviously convenient. Either the airport or Sun City information will have details of this service.

I have driven JNB to Sun City many times but the first time is definitely awkward as it is not a direct highway. However, it is very nice, broad, 2 lane hard top road.

But like most places in South Africa, driving is not recommended at night on account of danger from wandering animals and drunk drivers.