Firstly you will have a blast, Namibia/ Africa in general will spoil you for the rest of your life, and you will be permanantly broke from now on, saving every last penny to get back there!!! LOL
Some will say that a 300WSM could/would be a risky choice being that youre going to Africa, if some thing happens in transit to your rifle/ammo etc it could ruin your holiday, and a more "common" calibre would be a better choice like a 300WM or 338WM because if you do lose your ammo it is easly sourced in Namibia because its common.
Before all the doomsayers chime in, email one of the gun shops in Windhock and see if they stock 300WSM ammo, chances are they will, but it could help in the future.
Before you go get some factory stuff and find out what shoots best just in case you do need it, but just make sure the shop in Namibia stocks it too.
Now there will be plenty of opinions on this topic, and it is hard to put a exact bullet to the task at hand, any premium bonded bullet that shoots well in your rifle will do.
You can draw your own opinion of this but the 338 WM I used was shooting Partitions Zebra,200 yrds broadside, entered shoulder, turned 90' and came to rest in the brisket, it hit nothing vitial. It just pissed it off!! 458 Lott finished, it awsome hunt! and the bullet was a perfect mushroom but just didnt go straight
BUT on the 8 animals I shot that was the only "fail" or unusal event the rest were boom flops.
Even the Gemsbok @ 340 yrds the bullet hit perfect shoulder and exited other one.
If I was going back my first bullet choice would be 180 gr Accubonds followed by Interbonds., calibre chioce for plains game would be ..........300WSM!! for sure.
You are not hunting for the freezer HIT EM HARD!!!
PM if you want any more info on travel or things to do etc