That looks to me like it has had a re-trim at some point , the upholstery is much better than the paintwork , and that is unusual.
The underside condition is described as fantastic , and I don't think Williams would make this claim if there were any major issues. The chassis around the master cylinder is normally pretty well coated in oil and grease, and rot is not normally a problem there. Much more important is the bottom flanges of the bulkhead (it is a moisture trap) and the rear cross member. The chassis looks pretty straight and true to my eye , even in places where you do normally observe some minor kinking.
I would ask them to remove the trim so you can see the chassis rails from inside the car.
Take both rear wheels off and look up into the wheel arches, at the condition of the wing where it is bolted onto the wooden arch.
