Ftb: Damp survey in Victorian house

We have recently had an offer accepted on a ground floor Victorian flat and as first time buyers were very excited until the survey process started!

We had a level 3 survey come back with:
90% WME rear room external wall, 30–40% front room external wall.
High external ground levels and air bricks at ground level.
CCTV findings showing defective drains.
Blocked drains and gulleys.
But no internal walls have high readings or signs of damp and noted no flooring springiness.

Surveyor has now recommended we lift the floorboards to see any deterioration or rot to timber. It sounds unlikely the seller will allow us to perform the invasive work.

Has anyone had a surveyor flag likely concealed timber deterioration in a Victorian, and what was actually found when floors were eventually opened up? How expensive was any remediation? Or anyone had similar experiences?

We appreciate that older properties come with their quirks just want to understand more of the extend of what we’d be in for.