Hi,
I really love your initiative as well.
Regarding your first question, I have not heard of anyone who did this, though it really seems to be a great idea. Have you checked, that the bands supported by the FP3 are good for South Africa? You will find the supported bands in the Tech Specs under Network
As you are aiming at the Fairphone 3, there will be hardly an answer to your third question right now, as this model is being shipped right now and no long-term experience is available.
Hopefully, the demand for spare parts will be low.
Maybe you could order spare parts later, when the first failures occure. You could base that later order on your own experience as well as on the forum threads and messages. If there really happens to be some faulty parts, you then can order more informed not wasting money on stock, that will never be needed.
Though one spare part will always be needed: the display module. Accidents happen and glas does break, even gorilla-glas. I broke 2 displays of my FP2 for example and one of my FP1; so I would guess one spare display for every second phone would be a good guess (not everyone is as clumsy as me. 
)
You are aware, that FP does not deliver outside of Europe and you will have to buy from a reseller, I guess. You find a (maybe incomplete) list of resellers in this thread:
You will have to check, which one might deliver to South-Africa.
Getting batteries for spare parts might turn out a bit tricky, as they are considered hazardous and only few will send them abroad, let alone larger numbers. (Batteries inside phones are no problem.) That’s something, you might want to check beforehand, so your phone’s life will not be limited by that of the battery.
I have once done some research for even just sending batteries cross country in Europe:
I am wishing you lots of luck and success with your task.