Should the caliber start with a "4" or is a 375 enough gun? What about a 375 with solids? Or a 338 magnum with 250 grain solids or monolithic bullets for that matter? Double rifle or magazine fed? Will a shot that doesn't penetrate into the brain or spine still "turn" a charging animal? Will a shot with a 30 caliber that does penetrate the brain or spine still "stop" the charge? Open question, but particularly interested in buffalo.
A proper stopping rifle for a client is the one that is of legal caliber for the animal hunted with a proper bullet. After that it is proper placement of that first shot nothing more and nothing less.
For Cape buffalo a 375 is not regarded as a "stopping rifle", a 338 mag would not be legal. Solids particularly in the 375 H&H are not a good idea for Cape buffalo.
Buffalo unlike elephant are not easy to turn once they have started a charge you best drop him or face the consequences.
30 Caliber bullet that penetrates the brain or spine will stop the charge but only a uneducated fool would choose to use one on Cape Buffalo as a "stopping rifle"..
If you can shoot it equally well anything that starts with a"4" would be better than a 375.
Double or magazine? Personal choice, if you are the visiting hunter make sure you get a lot of practice, very few clients with double rifles I have guided where competent with them.(Meaning shoot it accurately, be able to take the quick second shot and oh boy....reloading it....)
Scoped bolt action is a better choice for accurate first shot placement, when using a double you may well have to pass up on shots you could have taken if using a scoped bolt action...
Unless you are the PH rather concentrate on premium grade bullets and first shot placement and less on stopping a charge, if you do the first part there will not be a charge to contend with and everyone walks away to hunt another day.....