As my little classification says , i'm a "newbie" with these cars , is this rust what would be expected with a car of this age , or did you get this one cheap because of it's condition??
I'm keen to get into one of these 6.9's , but your article has just made me re think??
Some rust is to be expected, unless it is a top example. The level of rust seen in Nathan's gold car is definitely not commonplace.
FWIW, my 6.9 was sitting unregistered in someone's front yard out at Ellenbrook WA, for a good part of a decade and prior to that, out at Kalgoorlie. All the screen seals are completely shot and the carpets were wet, but the rust is quite minimal for a W116, there are a few localised rust holes in the firewall and under the left rear wheelarch box-section.
What you don't want is a car with odd rust spots all over it... that usually means that the usual places are rusted out to an advanced stage.
Areas to check are:
Firewall, behind rubber sheeting at bonnet hinge area.
Boot wells, either side of spare wheel area under trim.
Around windscreen seals.
Bottom of doors.
Outer sill (some rust is very common in this area and not too much of a concern, unless the car is going to be getting wet all the time) and front mudguards.
Floorpan (hard to see because of sound deadening). First place to check is under the front carpets, in the far corner where the accelerator pedal mounts and on the passenger side in the same place. Water sits here where it enters the holes in the firewall/screen seal and eventually rusts out the floor.
The bumpers often rust out, but are easily replaced with good used ones.
Also watch out for bog in the above mentioned places.
Good luck Burty and I'm sure you'll find a good solid example... they do exist.

Tim