You and I are sort of in the same boat. Different business ideas, but both of us looking to make a change and do something we enjoy.
I'm a little frustrated at the moment as I was expecting to hear from a potential supplier today, but I'm not beaten.
So far I haven't had my idea met with much enthusiasm, but I'm not beaten.
I think you would do better to have a permanent place to do the changes, so you aren't a rolling oil refinery, but perhaps, with the correct insurance, you could pick up and return customer's cars and waive them the hassle of having to drop off and pick up the car. As far as mobile, what if you did mobile inspections and picked up business that way. Check oil, tire pressures, lights/signals etc.
Personally I think you should seriously give it a shot if its what you want to do.