[thstone]

If your goal is to try to address as many of the potential failure modes as possible, then Mr. Raby is probably one of the very best persons to consult. However, I'd suggest that you spend an equal amount of time learning about the failures and the likelihood of each failure mode as only you can decide if the cost is worth the potential benefits.

In terms of a PPI, it is rare to have anything invasive done because the owner (the seller) typically won't allow you or your mechanic to drop the sump or even remove the plugs for a leak down test. Thus, most PPI's are inspection based combined with a check with a PWIS or Durametric to check for any faults in the electronic systems.