Many guys take a .375 H&H out of nostalgia. However for plains game it isn't needed. Unless you want to take one. Plus as Ripp says if hunting DG sometimes it is good to have a rifle handling both.

I have used a .30-06 M98, a .375 H&H Mag M98, a 8x68S M98 and a 9.3x74R DR for plains game hunts or dual plains/DG hunts.

Something around a .30-06 or .300 Mag plus or minus is fine.

Take amoo with you. IF it gets lost you can try to buy replacement ammo if the cartridge is a readily available one.

Airline rules are usually you can take up to 5kg of ammo, packed separately in another bag from the rifle. Ammo should be in original cartrdige boxes or something like an MTM cartridge box. Individual cartridges should be separated from each other, ie by cardboard or plastic sections as in cartridge boxes or MTM boxes.

You will need to declare you are carrying a firearm and ammunition to the airline. They may want to inspect it. Some airlines require prior notice. You will need to declare you are carrying a firearm and ammunition on entering the African country. Some will want declared how many rounds you are carrying and inspect it. Rifle case and luggage with ammo in it needs to be locked when checked in. African hunting countries are used to all of this, but may be bureaucratic.

In Jo'burg the rifle case will be taken to a Police office where you retrieve it after getting paper work filled out. You will need the name of your outfitter and hunting property. Your outfitter can supply needed information for the forms.

If hunting in Namibia, if like me you need to stay overnight in Jo'burg, you may need to retrieve your rifle. I remember using a property name the South African based outfitter supplied to me to make it easier. Presumably some people do transit without having a South African property but not sure how they do it, or the SA Police just accept it.

Sometimes the Jo'bug airport police or porters ask for moeny (ie a bribe) for "beer money". I used to give them a US dollar or two. May not occur still or not, haven't been there recently.

The Afton Guest House used to be a good place to stay near the airport and had a lot of hunter who stay there. Louis, the owner, presuming he is still around, looked after his guests and would have someone meeting guests at the airport. Situated near to Jo'burg airport.

Out of the rifles you list, I would take the .30-06 and load it with good 200 or 180 gr projectiles.