400ltr tank i doubt would be enough a 650 ltr will be fine.
I have 2 people running off 650ltrs on some jobs all day and don't run out.
I would say you do need another pump if you get a flojet you don't need a varistream.
And 60psi is plenty unless you are using microbore then go for a 100psi.
Instead of having another DI vessel in the van why don't you just purify the water at home and transfer the water to the van, that way you don't need any di on board.
i will use more water, can i purchase a filtering systems which i can store in van , so i can connect to customer tap and filter more water
You can do this with a normal DI vessel but may use quite a bit of resin if in a hardwater area