I had some similar questions. Here's what I did: I run the show from my house. I redid my garage and went with a pro look. White walls, tons of lights, shelving, etc. I also made some friends with guys that were specialists in things like paintless dent removal, interior uphostery, tint, wheel refinishng, and paint. if a client comes to me with a problem i can't handle...I pass the buck. I tell the client that I can't do it but I have a person that I work with that is reputable in that specialty and they will be doing the work. I hae never had a client complain. I then pay my associate for the work he did and he shoots me 15% plus referrals. Ultimately, we should all work together and start a detailing shop, but I don't have the ballz right now. I'm still huntin down the remainder of my first venture.
I don't think I would start a mobile only gig. Personally, I like working at home and don't like travelling to locations. Too many variables. WHen I am not detailing I am web designing and doing other media work for businesses in my area, so having a car to work on gets me out of the house. I know, it's sad, but if not, I spend a lot of time in front of my LCD.