I don't have much advice for the business idea, other than it would be hard to build without trust/reputation. It'd have to start as very low-cost or free consultations that eventually build your reputation.
That being said, it probably would have helped me out ~3 years ago when I was buying my '98 328i. I absolutely love my car but at the time I overpaid for a car with near-blown (or blown) suspension, the usual dumping-fluid power steering system, and a leaky radiator. I don't regret it one bit, but had the car started in nicer condition, I wouldn't have minded [image: :)]