Bringing out the wannabe lawyer in me I'd say that the new owner of that ac should be entitly to get in depth answers from BHL about that overhaul. "privacy laws prevent us from disclosing what we did to the now your ac" would be rather odd.
However, typically the buyer wants to know before shelling out a fortune, what the preowner did to the machine and why. I'm afraid that this information can only be aquired directly from the current owner or from BHL if the current owner entitles them to talk to you. And if they do not allow BHL to answer all your questions, that'll tell you more about the ac than any workshop report, won't it
..then again, you could happen to drink tea in the very same cafe as one of the BHL wrenches...
I'm only a newbie PPL, that drools over used helicopter ads every now and then, and am seriously wondering how one actually accesses the price / worth relationship before purchase.