There is an SAA arrivals lounge in the international terminal, and another at the Intercontinental Hotel just across the road from the terminal building (which is a shared lounge which SAA business class can use) - refer to here for more on that.

I don't know what time the IAD flight arrives in JNB but early morning can be some what chaotic at passport control. If a few flights have arrived before you expect to wait anything up to 45 mins to clear passport control - but at least your bags should be waiting for you by then. Otherwise you could be first off the plane and first in the queue and not wait more than about 5 minutes.

It's a good 15-20 minute walk from the international terminal to the domestic terminal so leave enough time for that (including more time for queuing to check in - sometimes the Business desks can be just as busy as econmoy). If you want help with your bags there will be many porters in orange overalls offering to carry your bags (but make sure you have money first as they'll obviously want a tip - probably around R20 will do). Changing money in the international terminal wont be a problem - I think there is an ABSA bank there. Otherwise just use your bank card to withdraw money directly from the ATMs.