I went for a 650l two man setup, my flow controller is set to 27, some may say it's low but if you have never done wfp before then you have nothing to compare it to, if you are used to a high flow rate then it would probably feel a little slow. Anyway I have been cleaning wfp for 2 months on that flow rate and havnt had any problems I start at 9 and finish at 3.00-3.30 and have 30 min lunch. My work is domestic, the 3 bed semis are all close together then I have days when I am doing the bigger county stuff. At the end of the day there is always water left in the tank and that is with 2 of us using it.
I get the impression that those that have the flow cranked up are in there one so they don't need to worry about how much water they use, or they have the flow higher because it's quicker to rince so therefore they can get more work done.
If you don't want a large van then you are limited to how much water you can carry, the max in a connect/expert size van will be 650l, you won't have the payload to fit any bigger tank in.
You could always have a small trailer with 25l containers of water in for the days where you need more water