Are you perhaps overthinking this?
Most agree that oil analysis trends are important as part of an ongoing maintenance program. Most boats don't have them unfortunately!
Serial samples when available for a prospective purchaser also confirm that the most recent analysis is valid (i.e. the seller did not simply change all the fluids before sea trial to hide something)
An isolated pre-purchase normal sample is potentially reassuring but not definitive since trends are best.
The (presumably) one time pre-purchase samples you posted are valid to you the prospective purchaser because they are a red flag that you are going to have to spend some $$$ on at least 2 of the 3 engines. Use this information to ascertain the potential cost of repairs/rebuild/replacement (do this yourself if you have the knowledge base or invest in a mechanical inspection by a pro knowledgeable about those specific diesels/transmissions) Use this cost to either get a commensurate reduction in purchase price or walk.