Re: breakers
I agree with Mike. The primary function of a breaker is to trip on a fault or a sustained overload. A secondary function is to remain closed the rest of the time. A vast majority of the time, they are performing this secondary function.
You cannot assert that a breaker will perform its primary function when called upon to do so, without actually imposing an overcurrent and watching it trip. But that test could reduce the life of the breaker, and impact the chances that it would open on a future overload. And even if you did test the breaker and showed that it could trip, you still cannot say that it will trip next time. That would be to attempt to predict the future. IMHO, it would be a breach of professional ethics to make a statement that predicts the future.
As to providing a warrantee, such as "I will fix it for free if it fails," that can and should only be associated with a paid service. For example, "If you pay me for replacing the breakers, I will give you a warrantee against failures." But even that is not the same as issuing a guarantee that the breakers will not fail.
I think the strongest statement you can make solely on the basis of an electrical inspection, without replacing any existing equipment, is that the equipment appears to be in good working order, and that you did not discover anything that would violate any currently applicable codes.