You need someone to open up a section rather than guess, because siding base damage can be surface level or it can run into the rim joist and sill plate, and those are two very different numbers. Most rot contractors will do an assessment and give you a written scope you can hand to the seller's agent. That document is what makes negotiation possible. We did exactly this during our purchase and had GotRot out for a
Seattle Dry Rot Repair assessment; the written report is what got us a credit. Ask your agent to extend the inspection period if you need a few extra days. Get the assessment scheduled quickly, contractors book out and inspection periods don't extend themselves.