Welcome to AH,
What ever you decide to take, make sure that you are able to shoot it accurately. The first shot, help a bunch, when it is place in the right spot.
As for rifles, anything from 375 on up is capable of putting a cape buffalo in the salt shed.
as for rifle makers, there are many out there to look at and put your hands on to see if it is the right gun for you. Actually you are pushing the time limits on obtaining your rifle for a safari in July of this year. Not saying that it is not possible to do, just make sure you can shoot the rifle accurately that you select to take on your trip.
Take a mix of solids and softs ammunition along to use.
You did not indicate if you plan to take any plains game on this safari. Or if you plan on taking a second rifle along for that purpose.
Now in this is a one rifle, safari, look at the 375 H&H and or the 416 Rigby.