Develop a quick and effective diagnostic system, refine it as you move ahead. This way you can quickly inform the customer whomever they may be, family or otherwise, with a diagnosis and a charge for this service. At this time you can give them an estimate but what ever you come up with, double it. This way you weed out the serious customers from the cheapos right up front. When you have to stop what you are doing, research parts and prices, and then run out for the parts you spend a lot of time that you need to be compensated for.
My portable diagnostic box has compression gauges with adapters for 10-18MM spark plugs, a leakdown tester, several spark checkers, a capacitor tester, multimeter, non contact tachometer, inductive tachometer, an inductive pickup timing light and other misc items. I have a portable air compressor that I can take out on the road to operate the leakdown tester. I also have a small tip flexible light so I can look into cylinders. I also have two portable tool boxes the contents of which to a few years to develop.
But bottom line, you gotta collect money for your services. I charge a full hour or more for pick and delivery of a machine whether it is repaired or not.
Mad Mackie in CT