[Bungle]

I did WFP out of my car years ago. One thing I would recommend is for anyone going down this route take a DI vessel out with you. I used to ask customers if I could use their outside tap (of course) and fill a barrel whilst cleaning their house.

[Splash and dash]

£605 for a sack truck, barrel, pump & controller lol.

The stainless steel version is £1075 !

[james peters]

That back pack has a trolly , I have one . I have several back packs . you can buy a back pack on ebay for about £54 pound and it is just as good .... back packs are a great tool. i use mine a lot ( horses for courses ]..... you can get a shopping trolly for a tenner in home bargains , very light... just remove the shopping bag and you have a light trolly to put the back pack on .
in my opinion gardiner pole would be best.....
if your son in law is starting out with a back pack and barrels , water usage will be an issue . I switched to the gardiner blue pencil jets a few years ago. the rinse is just as good , but i use 30% less water.

hope this info is of use