The closest SAR locomotive out of a box would have to be the Mehanotechnica light Pacific, these have have been sold by various companies over the years including, AHM/IHC, Lifelike etc. The boiler and cab need a fair bit of work but the wheelbase and drivers are almost spot on to a 600 steamer though, many have been converted to 600 class steam locomotives in SA. The tender needs a lot of work as well.
It can be as simple as you want it really, you can go the whole way and get it near enough or just do some things and a quick repaint. It is up to you.
As this body works for a 600 steam locomotive, it is also offered as Mikado in these ranges as well, so it could be assumed that you might be able to get a 700 class type loco out of one, either a 700 class, 710 class or a 740 class loco. I have never done one of these locomotives though, I have done a 600 though. This would probably need a new body for the locomotive made.
Note that the valve gear on the pacific at least is Baker valve gear which was only used on one class of SAR locomotives the 620 class, so it might be able to with a lot of rebuilding to get a 620class out of the pacific as well, you would need to build a complete body for the locomotive though from scratch. I have done one, a long time ago and it will pass at a glance for one, just don't count rivets though.
All the above will only be near enough's though, the 600 class being the closest to scale of any of them.
I used some PVC plumbing pipe of near enough diameter as a starting point for the boilers though, this solves a lot of problems and is easy to work as well.