Hi
Mainley with S-types its corrosion, first sills, hidden by plastic covers can completely rot away, dont matter what year, also check rear sub frame, rusty rear arches usually means there not a lot of the sill left
diesel ideally service history, lots of regular oil changes are best, leaky injector can contaminate engine oil and cause the common snapped crank.
check brakes, servo are a really common fail on the diesel, can be expensive to source a good one
check gearbox is smooth and it drives nice, lots of bushes on the s-type and gets expensive if they all need doing, check ac, make sure both sides get hot and both sides get ice cold. check electric park brake just keep applying and driving.
early diesels did not have dpf's and had a lot less problems.
cheers
Joe