3top,
I am not sure the ERC kit can be fitted on an Alouette III. Anyway the 2250 kgs MAUW cannot normally be exceeded, ext load or not. From the figures you gave, the basic system should do. I understand you are going to fly very short trips. In this condition we used to reduce the max payload by 10%. It will be more than compensated by better speed and easier manoeuvering, meaning shorter trips.
Be smooth on the collective since the Astazou is more sensitive than the Artouste - and burn less fuel.
Cheers
ATN