I use a Ro-man 450 gpd ro, with a booster pump, this ro is very efficient.
It takes my tap water down from 400 + on average down to about 5, so no need to buy di resin.
It would make your 400 ltrs per day in around 8 hrs ish, maybe a bit quicker.
If you dont need to make water super fast, this is the way i would go.
Slower but better quality ( plus no di expence )