I wash cars with Protect All Quick and Easy Wash, so I only need about a gallon of water per car. I have a pretty good 12 volt vacuum (Campbell-Hausen), but prefer to use my big wet/dry vac. Most businesses now have electrical outlets outdoors, so I don't have to run a cord under the doors. I haven't been able to find a quiet enough generator that is also small enough to fit in my trunk. I've seen one that is suitcase size, so I may look into that.
As far as dust goes, I have my California duster with me and on windy days, I do have to dust after washing and before waxing. Just part of the business.