No money in doing cars as Daz said.
It's going to take you at least 10 minutes, maybe 15 to do it thoroughly.
In that time you could do a £10 house, and who's going to pay £10 to have their car washed?!
And no I wouldn't use wfp on one, now way!
Yes some dealers use pure water, to RINSE IT OFF! Not to scrub it!
It removes the polish, so I hope you're doing that afterwards too!
And don't say it won't remove polish, because otherwise it would be pretty useless at getting grease off windows wouldn't it?