There are two types of petrol driven washer you can get. Direct feed (lower LPM & PSI spec type) that are designed for light use only and heavy duty washers that draw water from a source be it a wheelie bin, tank, etc that have a higher LPM flow rate & PSI in general. However don't be confused with the cheap china ones being sold new on ebay for about £200 that so say can draw water as are crap and the pump will not last five mins.
Regards to hoses etc direct type feed washers you just connect it up using standard garden hose reel and conections directly to an outside tap. However in doing so you are dependant on the tap water flow and pressure if the washer pump will work properley at full pressure and flow rate or not. Or if using a draw type washer you run a standard garden hose reel to say a wheelie bin so the washer is fed from that using the draw hose and weighted filter that came with the washer. Most would also fit a ball valve to stop the tap water overflowing.
If wanting to use van tank that you use for pure water for wfp work then is not a great idea sticking tap water in it just so to run a draw type washer if you ask me. Not unless you already do so as you're lucky enough to live in a very soft water area and di on demand straight from tap water in the tank to your wfp pole. However if doing so the draw feed hose from the tank needs to be at least 1/2 " or 3/4" so the washer pump is not starved of water. Then you are really looking at building a van mount pressure washer system with fitted pressure washer hose reels etc rather than a portable system.
What sort of budget do you have as to get into pressure washing to buy decent kit is not cheap even buying good used kit?
Also unless you can sell the service at a decent price to your existing domestic customers then may not be worth getting into pressure washing at all as you could probably make more money on the glass.
I had one job cancel yesterday because the lady spoke to next door that said you can just buy and spray a chem on her block paved drive instead and mentioned to me in an email it was the more trendy way to do it now. Good luck with that one I thought as was in a terrible state that had not been cleaned in years.
It don't help when people google vids like this as they assume it's an easy and cheaper way to do it themselves rather than paying someone to get the job done properly.
http://www.smartseal.co.uk/help-advice/step-by-step-video-guides.htm