I'm doing much the same sort of thing this September. To fly in SA on a JAA PPL you have to take an open book air law exam and then a validation flight. This is basically a GFT plus a short triangular nav. trip. You then have to get your documents to the SA CAA and get issued with a SA PPL. This can take one to several days depending on the club you are flying with.
Irv Lee can do SA PPL revalidations in this country, but when I asked a few weeks ago, I was told that he cannot yet do the initial validation before you go. You have to do that in SA.
I am doing my validation and hire with Algoa at Port Elizabeth. From my dealings with them so far I can tell you that they are friendly, helpful, and very, very efficient. Why don't you talk to them - they will clear up the details for you.
As for the jabs - as far as I can see, you don't need any unless you are going to a known malarial area (basically the national parks in the north), or you are coming from an area with endemic yellow fever (not the UK!). But don't take my word for it - check with the foreign office web site.