like already suggested. QEW is a good choice.
Another choice is just go ahead and use a bucket and water from the customer's house. A lot of areas won't bother you about it at all as people wash their own cars in their driveway anyway.
Since you're getting a truck, then another option would be the mount water tanks and a reclaim system.
It all depends on how far you are going to take this. If this will be your full-time business and only source of income, then I suggest doing it right the first time. A weekend or suplemental income job, then it's your choice. It'll take a lot longer doing weekends, small jobs to earn back the money you invested so keep that in mind.