Just for polishing after the RO, a 7ltr DI vessel doesn't quite fit in the budget, but I was thinking that the 700ml that they sell at purefreedom would work quite well? Any idea how long it might last me on a 400ltr tank? I haven't quite figured out how to do the TDS calculations yet.
(Ofcourse depending on how low the RO gets it, but just a rough idea, is it worth going with the 7ltr DI vessel?)