I’m not sure why you would trust the opinion of a group member rather than your own instincts and opinions of the expert mechanics? You seem convinced the boat has an issue and I seriously doubt even the most knowledgeable members of this group can guide or are willing to guide you down a particular path.
If this is the boat for you, then reach a deal with the owner. Or, cut your losses and look for another boat.
With best regards,
Sailing Vessel - Cream Puff - SM2K - #275
Currenlty cruising – Turks and Caicos Islands