Inventory isn't that bad, sure you need lots of stuff but it's manageable
In past times, I maintained an airworthy parts stores, for fabrication I did for specialty aviation customers. I had the advantage of nearly always knowing before I needed it, what I'd need, and some extra budget to stock a little extra of that, and some more extra around the edges (longer and shorter bolts and rivets, for example).
Planning and stocking it was one thing, maintaining and documenting the required traceability was a big job on it's own! I was required to provide the certification documents through, which I did. My clients were prepared to pay the shop time required to do that, it was not none!
The other thing which I have seen omissions in has been aircraft specific maintenance check sheets. Yeah, I know you can just refer to Car 625, Appendix B & C, but you still have to account for the ICA which could be required for every mod on the plane. I've been the holder of several Canadian and FAA STC's since 2002, and I get several calls a year asking if my ICA, at revision A, is still the valid version. "Yes, it is." "Thank you, just checking...". An AMO doing their job properly.
For the owners who want to "help", well, their attitude is in the positive direction... So direct them to the paperwork that they can do before they bring you the plane. An airplane specific maintenance checklist, which details every task, the standard or reference for each of those tasks, and all of the applicable ICA's at the correct revisions, and, a recent AD search. I find that by the time the owner has actually understood, and documented these properly, they begin to appreciate why proper airplane maintenance costs what it does, it's not just wrench turning.... And, they're probably exhausted, so they'll be happy to just give you the plane, and go on about the rest of their weekend with their family!
