Firstly, nice looking truck. I've had 3 pickups (not for cleaning) and my last was a Toyota Intimidator.
I also used a pure freedom 25l trolley for a few years. Everyone is different and you need to go with your gut instinct but if it were me I would:
Fit a tank (400 litre ish). I would strap it in (if that's your garage then you can take it out weekends to put other stuff in the back of your truck, for when you take the kids out).
I would not bother with a trolley (too expensive). I would make a pump box (battery, pump, controller). This will take up room but again you can easily remove it at the weekend.
I am guessing you want a family vehicle at the weekend?
If you have awkward access then use 100m of pole hose (gardiners reinforced) this will take up much less room on the reel.
As for you custies not wanting water fed pole I would do 4 things:
1. Learn how to clean properly with a wfp
2. Give them some old crap about health and safety, or not even mention it.
3. I always tell them I will leave their windows 'SOAKING WET'
4. Always, always, always, on a first clean, get the customer to come out and see how the windows are drying. If you start in the sun, by the time you finish, some windows will start to dry. Bring the customer out and show them the part that has dried.
I always say 'you can see that the window is still drying, but look at the top part that has started to dry! See how clean it is! When all your windows are dry, they will look like that.
5. OK so I've added 5. On first cleans, on the lower level windows, wipe the underside of the small top openers. Also wipe fully the window seals. This gives the appearance that the windows are not as wet as they appear.
Get this little routine correct and it will work every time.
Good luck. Be positive, it's your business.