I just checked the water from my water butt and it shows a TDS of 42.
Would that still need running through resin to prevent leaving marks on windows, how low does it need to be?
42 is still too high but that could be more cost effective to just run it through a di vessel , however ideally you would be better off having the rain water go through something that could filter any large sediment/crap thats in it (even something like what tights are made from) stick it on the end of the down pipe or whatever is going into the water butt. However i would still look into buying it from another window cleaner they may not advertise they sell it but i bet there is someone out there who would sell you some now and again.
Or just keep your eye out for a second hand 300gpd ro system would be cheap as chips, stick some new membranes in it and it will last you a long time.
Pity you dont live closer to me i have an old 450gpd r/o system with booster pump and auto switch off and all the pre filter housings that i dont really use anymore.