A passport( with at least 3 months before expiring), Money to pay Namibian Road Tax, Car Papers and a ZA sticker....
When the the whole concept of country stickers came out more than a decade back they were quite strict about it..... These days it would appear not as I have seen multiple cars without the sticker, so I think the novelty wore off, but one can never say....
No Meat, Poultry or dairy products allowed across the border due to Foot and Mouth disease in SA....
Yep! That's about it....