Tough one, this. Eddie Barlow and Trevor Goddard formed a formidable partnership in the '60s, and while Barlow was the better pure bat, Goddard was a superb all round cricketer. I'm picking neither though.
Bruce Mitchell was a wonderful servant of South African cricket, and for many years the country's all time leading runscorer. Graeme Smith, the modern champion, is an absolute beast of a player and would bring great leadership the side. I'm picking neither.
Who am I picking? First up it's Barry Richards. Yes, I usually try to base all time Test XIs on performances in Test cricket, but there are simply too many good judges who played with and against him and saw him close at hand who testify to his greatness for me to leave him out. I'm not one for blindly believing the hype, but when such praise as Richards gets is damn-near universal, it's hard to ignore.
My other pick is Herbie Taylor. Because he defied Barney when Barney was at his greatest, because I don't think he'll get picked for the middle order against Pollock, Kallis and Nourse, and because he didn't even get a mention in the OP.