Easy to say it may not happen but chances are with a unit not designed for use to this degree it will fail and it will cost this guy money, he can get a unit that can produce water quicker... You must see my point on this m8, I wasnt knocking what you was saying, but I do not think a 100 gpd ro is good enough for a window cleaning business, buy that cheap and you will buy more than twice costing more money..
my point is though, he uses 250 L a day, thats what the 100 GPD puts out, so he doesnt really need much more
yes i would have 1 days water stored as a backup, he can do this in the cellar
cheap doesnt allway mean you buy twice ? ? ? he of course might find he needs more water, so he can add a 2nd 100 GPD membrane to the unit ( but thats a option he has if he needs it )
You Matt will know aswell as me that when you start you use a lot more water, so if the guy thinks 250 will be more like 350, adding another membrane incase is my point, the cost on these second hand is peanuts, why get a 100gpd and be at max straight away, even if money is tight, which I understand with new baby on the way, for the cost of maybe £10 more you can get an ro that does not need changing shortly after buying. that is what I mean when I say buy cheap buy twice, you have just said yourself that adding another membrane 
Ian
now i dont want to appear argumentative, BUT a extra 100 GPD membrane will cost more then 10 quid
the risk with second hand RO's is that the membrane and pre-filters will be knackered, i would never buy 2nd hand, unless you price into the deal the cost of new membrane and pre-filters
so i am afraid its NEW for me, then
your idea of a 300 / 450 RO is alot more money
a 200 GPD is 200 quid
300 is 280 quid
450 is a few quid more at 290 ( ish )
i agree with you if he was thinking of getting a 300, he might aswell get a 450
but the price jump from 100 upwards is more than 100 quid
when i started i used 150 L per day, its only now that i use more, as i do up and down WFP ( which now means i use 250 L a day ), he does bottom the trad way, so i guess the 250 is at the top of the usage chart ( as it stands )