Ok,.. let me get this right,...
Problem: Lots of windows spotting, custies will be unhappy, might loose work
Solution: Spend lots of money on gear that may not fix the issue

Seriously,... you can use more water with your trolley system. It'll mean changing barrels more often, but it'll let you know if that will help without spending on a van mount!
Your problem is most likely technique,.. not equipment or water flow.
My advice would be to buy a 2nd TDS meter so you can double check your water is ok. Increase your flow rate to 2 litres per minute ( a plastic jug & a stopwatch will help you set this), and be patient. First cleans are often a nightmare & need a double clean to come right.
Figure out what your issue is first, or you may just be wasting money!
A van mount is a great bit of kit btw,.. not knocking it as a viable ambition,.. just get the spotting sorted first.