I would really do your homework before you start this "business". You are opening your parents up to huge liability if you damage someone's car.
Do you have insurance, a business license, water reclamtion mats etc.? Do you know how to use that buffer you are going to buy? Do you know the difference between a polish, wax, and paint sealant? Do you know the difference between a rotary and orbital buffer? Do you know what kind of polishing pads and towels you'll need to use so you won't leave scratches all over someone's car? Do you know what chemicals to use on the windows so you won't leave streaks all over the windows?
You do know that you can't just dump the waste water on the ground. Since you are operating as a business the EPA mandates that you must capture all your waste run off and dispose of it in a sanitary sewer. You'll need to spend about $750.00 on a water reclamation mat and find someone who will let you dump your waste water in their sanitary sewer. You'll also need a container to transport your waste water off site. Hack job mobile "detail" businesses violate this law all the time but if you get caught you could be looking at thousands in fines.
I am not trying to be critical, you just need to know what you are getting into.