I'm NOT a shop owner however, I thought I would throw in my two cents worth. I'm not sure what you presently do for a living . Myself, I have a job that has nothing to do with cars, but I have a couple of cars, and projects that I work on as a hobby.
I love it - come home from my (non car related job) and go out to the garage, work on my project, while the speed channel is blaring away on the TV in my garage. I fiddle and do what I enjoy ( working on cars). When I'm tired I close the garage , clean up and go in the house.
NOW my point is: as a hobby I throughly enjoy working on these old cars, however if I had to work on cars everyday for a living, for other people, within their time frame, schedules and dead lines, with a quality of work and unrealistic expectations from some of the customers, then soon my love of the hobby , may soon become hate for the work, resulting in ruining both my hobby and my work.
The only reason I mention this is because sometimes people such as myself have a hobby that they think would make a good business, however remember that if you turn it into a business, its no longer your hobby; its your job and you a no longer working for yourself; you are working for your customer.
And re-read Bob Parameters answer above: I agree with him wholeheartedly - you need to know more about how to run a business then you need to know about cars.
Good luck with whatever you decide - Get someone you know and trust that has an existing sucessful business and have them help you create a "business plan".