Everybody is right on the larger tank, thats what I also decided. You can fill it and it ensures you dont run out of water in a day, regardless of whatever jobs you have planned or unplanned.
Im a diy er and you mentioned you already produce your own water. Therefore all you need is a tank secured in the van. A pump ran of a leisure battery feeding into a hose on a reel and then your pole n brush.
Really, the only ones who really care whats in the back of the van is us fellow window cleaners. So it depends how much you want to spend on setting the van up as to which way you go about it and perhaps your own diy skills. (However its not that hard to do, just depends how neat and fancy you want to do so) i would check out you tube videos to give you some ideas. Myself and others on this forum have shown theirs on youtube.
The size vans your looking at are ideal for a one man or even a two man band. So it gives scope for expansion but also in work as well. For instance you could easily fit a decent pressure washer in the back as well.
I use the S type hose reel from pure freedom which is robust and handles smoothly enough. I would definately recommend electric, again be it diy or factory bought. It simply makes life easier, as simple as that.
(Im no expert as this vid will testify to, but it shows how easy you can do an electric reel. (mine cost under £60, for all the parts n connections to do so)
https://youtu.be/lqzJnoOjluc ))
Its also easy to upgrade to a 2 man system from a one man set up. The yorkshire man inside me would say, yes your van can handle the weight of having it set up for 2 man ie 2 reels etc. But why would you spend extra in doing so when its not needed and can easily be added later.
Yes a spare set up so you have no down time if anything goes wrong, but in the 3 yrs or so since i went wfp, i havent come across a pump or hose issue that cost me too much time to quickly fix. As the essential kit you need is a small tool box of spares. Spare ptfe tape, spare o clamps etx just to address the little issues that may come up, but not often do.