Well I tried tha aircon thing but to be honest it does not work too great and cost abour R250 to run for the month and you still get temp swings. I would go the chiller route and have it outside so it does not heat up the room - also a twirly would help a bit...
That is the issue with the aircon, it doesn't allow "pin-point" accuracy with regard to temperature.
The room will be insulated, with polystyrene on the walls, and "think pink" stuff in the roof. I also reckon that a water volume of somewhere between 4 and 6 thousand litres will take a while to rise/fall in temp.
So if you use two of them, it's 3kW.
3kW x 24hours x 31days = 2232kWH
At about 50c / kWH the worst case cost would be about R1,116 / month.
But ofcourse it won't be on all the time. Above is worst case.