Pending the trolley system, yes you can adapt later into a van system if required. Or alternatively if using a diy system (pending tap tds, filter water into an ibc tank in storage and transfer water into tank in van. Minimizing van space taken uo with an upright tank.
I switched from traditional cleaning of 14 yrs last yr to wfp. I too was considering using trolley for upstairs and trad downstairs and later adapt to van system. I was adviced by many to skip this procedure as many end up doing tank in van. For me it paid off taking that advice. Because to be honest, i fills me with dread messing around refilling trolley.
Today i also have a trolley for some jobs which are too awkward to park etc. Trolley is great (gardiner back pack system) and using a valve tap, i can clean 3 -4 houses before it gets filled, thats full houses.
I use a fiat scudo and also pressure wash. My reel n brushes come out and my 500ltr upright tank stays in van and i still have space for p/washer.
But if wanting to start cheap whilst this side of business grows, then diy the gardiner back pack, a carbon pole and a pure freedom ro system into an ibc tank or double di vessel pending your tap tds.