I will look cheesy and sound even worse to our golden ears, but outdoor horn speakers (usually metal or plastic, thing classic bullhorn style) will probably be the most durable product for your needs. Replacement speakers of similar design -- should they be needed -- may be more available in Africa than other styles.
Based on the reports from my church's missionaries in Africa, sound quality isn't as important as we believe it to be here in the First World. The first objective is to spread the Word of God by any means possible. As for worship, the Africans join in worship very enthusiastically to the point where the PA system nearly gets drowned out. So if you're just using it for a songleader or two, maybe it's not so important for it to have a perfectly flat response.
BUT.... your experience may vary. If you can find a good quality system that does what you want at an acceptable price, go for it!