Does he live in a hard water area or soft water? Test the tds. If its over 100 he needs an ro system. If its under he could potentially just use the double di method.
How good are you both at diy? Rather than buy a "system". I'd put a tank, pump, battery and hose reel in the van and save a fortune.
Other options are just putting 25L barrels of pure in the van and using a trolley system or backpack on a small sack truck. These are even cheaper.
I only do domestic windows now if they are part of a whole exterior house clean so ive got rid of my system and all i take on the van are barrels of pure and two backpacks. (I always carry a spare incase one breaks down).
This is where you cant skip on money imo. Get a full carbon pole. I used to carry a 22ft and 27ft slx but now I just use a gardiners slx 27. It covers all bases domestically. Carbon poles are less whippy. His body will thank him for it. A must have for the pole is a univalve. It saves a ton of water. Especially if he's doing tons of first cleans as he will go through a lot of water to start with.
Unless you are doing what Daniel does get a tank....I spent a year on trolley and barrels and it's hardwork and slower......I stripped the trolley down and used the controller, battery and pump with the tank.....100m hose and reel and as Daniel said get a decent pole....SLX27 is ideal.....I also have a CLX for other exterior cleaning jobs saving my SLX for windows.....apart from the water purification you'll get a good set up for a lot less than £2k.......if you can double di like me about another£240 for 2 x 16 litre di's.