Depends what payload it is
Start with a 250 litre streamline system
You may even be able to go bigger if you have one of the higher payload Berlingos.
Also get yourself a TDS meter to find out whether you need to RO & DI (TDS above say 100ppm) or just DI (less than 100ppm).
Quite a few suppliers around including Grippatank, Pure Freedom, Window Cleaning Warehouse, Pure2o amongst others - I suggest you 'phone around & see what deal they can do for you. Alternatively get the bits & install your own system (this is what I did - apart from securing the tank through the van chassis which the local garage did for me).