I actually did research work on this topic, not so much how they prepared but to see if they really feared a race war. The problem is a lack of material. The majority of people involved don't want to talk and before the government changed over they actively destroyed a lot of records. I went to the national archives in Pretoria to find stuff and came up with only three documents, you can actually see there just how much is missing.

Anyways, their main policies in dealing with the situation sought more to avoid war than fight it. Botha actually implemented a lot of reforms to apartheid policy to this end, such as the establishment of a non-white parliament (PW Botha, The Last Betrayal by Eschel Rhoodie is a good source).

There's not much academic material because it goes against the good vs. evil narrative that everyone likes (i'll see what i can find and add it here but some of the books written by Frederik Van Zyl Slabbert refer to it quite a bit), but a contributing factor to the fact they started talking to the ANC in the first place was that both sides feared radicalization of the black population.

There were of course the more violent preparations too. The government destroyed most evidence before the ANC took power but their are mentions in TRC records of things like trying to find a biological weapon that targets only black people and so on. They never really got anywhere.

They also worked quite hard at arming the white population. Towards the end they were selling military surplus rifles to anyone who'd done national service for stupidly cheap prices and had youth camps where kids were taught field-craft and so on.