Capitec does seem to work out cheapest.
I'd strongly advise against relying on one card though. I'm carrying a couple cards - that's not me being fancy...just noticed that if you travel enough you encounter places that don't accept a specific card. (e.g. No Visa :wtf::wtf

. And since I don't carry cash that can be awkward as hell.
The other thing to keep in mind is that some places don't like foreign cards. A visa card is supposed to be a visa card anywhere but in reality it doesn't work that way. Take a look at the next ZA toll road you drive through...there is a sign there saying RSA cards only. Same thing applies in reverse.
Also...since it's a business trip
talk to your boss in advance & tell him you'll be expensing this at whatever is on your bank statement, not the invoice amount. I got a number of claims like that from my underlings...people travel and got absolutely fk'd on the rates but hey the business demanded that they travel so the business picks up the bill...incl the getting fk'd part. Point is you might want to mention this in advance.
Didn't need to do that with FNB.
"Need" is tricky here.
Most transactions should go through without alerting the bank in advance, but it's safer to do so anyway. They can actually adjust this on their systems - the details of what they adjust seem to vary & they don't all adjust the algos but as a minimum they seem to leave a note saying "Havoc goes to US"...so it gives the fraud dept something to work with. With ZA cards I certainly would...they seem rather sensitive...my dads SB gold card got flagged & blocked for foreign tx within a day so ended up handing him one of my cards.