Would like to interject a couple of thoughts here. My son took a NEF handirifle, .30-06, for our plains game hunt, in RSA, in the summer of 2008. Tim posted the link above. Absolutely no issues with the handi for plains game, anywhere in RSA.
That being said, I think you have 2 issues facing you in your plans for 3 rufles/1shotgun for RSA. First, the airlines have specific weight allowances for ammo, on the plane. Of course you could put some of it in your wife's luggage. but 5.5 kg[I think] is the maximum weight of ammo you can take per person. Secondly, I think you are only allowed 3 guns per person for import. Again, doable, if you list some under your wife. Don't know how that works with 1 receiver and 4 barrels. In any case, I'd buy your shotgun ammo over there, and only take rifle ammo with you. You do understand that you will have to do a customs form 4457 on each barrel to get them back in the country, coming home. I hunted in Limpopo, with my son, on our hunt in 2008, but have read a lot about the eastern cape. Why not take a .30-06 with different weight bullets, rather than 3 different rifle barrels, with 3 different types of ammo. The .30-06 with loads from 110 gr to 220 gr. will kill every head of plains game from steinbok up to cape eland, very efficiently. The .45-120 is a great idea, but not neccessary, and the cartridges weigh more[airlines again]. I took a .45-70, when I went, and it flattened everything I shot with it.
All I can say is I like youe style, but I would check the logistics out to keep the trip fun, and not a monumental PITA. JMHO.
Mad Dog