What will you be aiming at? If DG is excluded, I would look at the 300H&H Magnum or 338 Win Magnum. If recoil is not your thing, get the 300H&H. My wife has one and she has taken just about anything below the DG category with it; Kudu, Oryx, Impala, Warthog, BWB, all with single shot kills. Distances 50-300 meters.
If you don't mind a bit of recoil, get the 338WM, it can also be used to take everything below DG. I shoot it and it has not let me down yet.
If your sights are on DG as well, then 375H&H would be the minimum calibre for SA. If memory serves me right and you hunt Namibia, they only have an energy restriction on DG of something like 3 980 ft-lbs. If you have a VERY HOT loaded 338WM 300 grainer or a 338 Lapua Magnum, you should be good to go
However, just because 375H&H is minimum calibre for DG, it should not necessarily be the go-to calibre. There is much to say for knocking a charging buffalo on his ass with a 416 Rigby.
Most important, pick the calibre that you are comfortable with. Can handle really well and shoot accurately. If you are comfy with the HOWA dirty-ol'-six and DG is not on your list, take the -06, it is more than sufficient for anything you would come across in SA (except DG), especially bush hunting where you would be lucky to get a 250 meter shot in. Most shots will be 40-160 meters. If you will also be doing some plains shooting (as you have mentioned), the 300H&H Magnum or 338WM should serve you well. My Father in Law is a one gun man and has taken a Kudu at 450 meters with the -06. So even the -06 will be more than adequate for plains shooting.