I think the majority of Commer owners in the other hemisphere are more concerned with fitting heaters.
Mine's got 240 air, 2 windows at 40 MPH
But seriously, there used to be plenty of companies doing aftermarket underdash air conditioning that would work fine on a Commer. As there is nothing to intergrate air conditioning into on a Commer you have a blank canvas, though the first thing I would be looking at is some heavy tinting on the windows so when the air is fitted it doesn't have to work as hard to keep the large cabin cooled to something comfortable on a hot summers day.
Cheers Stephen