SAA's A319 figs refer.
MTOW-70 000KG
MZFW-57 000KG
MAX FUEL-18 000KG.
SEATS-120
You will not be perf limited in the 319 (even at JNB at 5500' at 30deg). 108 pax will need a ZFW of approx 53000kg. That should leave you with 17 000KG for fuel.
According to FCOM 4.10 for a 3000mn flight at FL370 you will need 13249kg of fuel with a flight time of 6hr 51min. There are lots of alternates either side if the pond so diversion fuel should be minimal.
Hope that helps