my tap water is about 75 ppm, after merlin about 1ppm, i change my prefilter every 3 months, membranes are £150 so i think it would cost more if you didnt change the prefilter regularly.
my 650ltr tank takes about 6hrs to fill so i think yours is ok.
my resin barrells are after the tank maybe that would be better than straight after merlin?