Bill:
You see thats the problem, you have one man who is going to be wearing at least 4-hats,Electrician,plummer, Hvac, Carpenter, and should know the trades as well as we do.
Put another way when it comes to plumbing I can fix the bathroom tub fawcets, or make a Bookshelf(almost).
What to tell them ? Read and learn all you can in order to do a first rate job. I think there should be some type of oversight panel, under the auspices of the local building department, where if there is any doubt in the Home Inspectors mind at all he has someone to fall back on for help .
It seems to me that that is an awful lot of responsibility for just one person and on his say-so alone a family will base their judgement, just to find out that , oops he made a mistake.
He should be very broad based, know all the fields he is inspecting, that in itself is a tall order, for anybody.

-Mark-