If I was starting wfp again, given the smaller demand for pure that I had back in 2010 I would have gone straight for a 300gpd system in to a 600 litre ibc.
The system would always be running to keep the ibc topped up, and I wouldn't find myself scrimping on water use on jobs. If you find demand increases, just stick a cheap booster pump on, it should provide that faster fill for the times of higher demand. The 300 gpd is more likely going to be closer to 100ish (still around 400 litres) as everyday conditions are usually very different to the projected output.