Hi. There are 3 flights at that time and one at 16:25, so if something goes awry you'll be taken care of, and May is not the season where things would be socked in all day. (But you are breaking one of the standard SA “rules” to be in the departure city the day before for international flights.)
Nope, you do not have to arrive early for check in. But I would suggest that for sure you do electronic check-in 48 hours before.
Since you have an afternoon departure, you might want to think about doing an anytime luggage drop in the city, a new service offered by LAN.