There is also document duplication through email. Even with perfect platform versioning, attachments create detached copies. Links can reduce that, provided recipients have appropriate access and revoked access does not obstruct the retained history.
At this stage I’d carry around 15%, then reconsider it after opening-up inspections and a detailed scope. Ask whether the price includes access panels, removal and disposal, reinstatement after investigations, testing, temporary shutdowns and work arising from failed electrical checks. Also...