I was told and read that in a hard water area output would be low with 300GPD. What would you recommend for a static system then?
I will have 500ltr in van and use 250 a day.
Nothing wrong with a 4040 if you may want to expand the business eventually. 300 gpd ought to be sufficient initially but they don't give good results in all situations. I was getting 450 litres a day from mine - and that was with a booster pump. There were odd occasions, when I was working hard and needed high flow rates for first cleans, that I was in danger of running out of water. Typically, you should get rather more than that though. A decent compromise between a 300 gpd and a 4040 would be a Merlin. I have one of those and it would give me a bitr over 1000 litres a day (without a booster pump) if I had the storage capacity. They are said to be wasteful of water though so may not be your best option if you are on a meter or in times of supply difficulties.