Yes, wheelie bin is good for this. Have outlet connection at bottom to pressure washer, fit an inlet ballcock valve, and return to tank connection at top. I use a small wheelie bin only 80l and is fine providing customers water pressure is ok - but this is the same problem whatever size buffer tank you use, if not taking a large tank full to each job. Wheelie bin ideal for storing hoses and lances on the van. I have similar spec pressure washer to you. I have had van mount and trailer mounts in the past eight years and prefer the flexibility of being able to move the pressure washer.
Neil