If financing is involved, condition and property type can affect what a lender will accept. That is worth discussing with the relevant lender before treating every dated property equally.
Then ask each contractor to price the normalized scope and list unknowns separately, with a written process for approving changes. For sequencing, I would want investigation and any permit questions resolved first, followed by hidden electrical/plumbing work, moisture treatment, bathrooms and...
With exclusions still moving around, I would provisionally carry 15% and be prepared for 20% if plumbing routes, electrical condition and moisture cannot be inspected properly before work starts. The percentage matters less than access: ask what can be opened or tested now, who pays for making...