Rust is a problem that needs looking at when buying ANY classic car. Rust high up in the firewall, on each side is a place to go looking, as in any W116.
Other than that, have a good look for rust in all the areas you would look for in other cars, especially around windows, etc. Have a look for splitting , that may reveal evidence of 'bogged up' fibreglass or putty work under the paint.
The water pump is an achilles heel for a 6.9, and is specialised to the 6.9. A reconditioned replacement is the normal way to go. The water pump can be checked for noisy operation, by using a very large screwdriver as a stethoscope. Keep the screwdriver WELL away from the fanblades!
The hydraulic fan clutch should be checked for correct function, if the engine runs hot.
A compression test should be carried out if considering the purchase of any classic car. Check on the condition of all hoses and belts, also common for any car. Good idea to check the spark plugs when this done.
Check for exhaust pipe smoke.
Need to find out when the timing chain was last replaced, or at least to investigate if a replacement is required. That is usually done AFTER purchase, unfortunately. Sometimes, the previous owner will be able to provide you with that information, and evidence via receipt, if the previous owner has had it done.
Look for oil leaks, especially in relation to all suspension components, under the gearbox, around the rear main seal, and in the area of the diff. Listen for diff noise when the car is on the road. The front main seal is also a source for leaks, if the oil seal around the harmonic engine balancer needs replacing.
When the car is on the road, listen for creaks and groans. It could mean the front suspension bushings need replacement, or perhaps the ball socket mountings for the front struts. With that great chunk of 6.9 cast iron engine over the front wheels, the tyres and front suspension have a lot of work to do, especially it the car is fitted with low profile tyres. Look for even tyre wear, correct wheel alignment with these beasts is essential.
Check the gearbox for smooth changes without flaring, and check the gearbox oil, for a burnt smell.
I would guess that you would already know about the suspension, and if it allows the car to sag or not. If it does, it needs to be investigated.
Check for correct operation of the pneumatic locking system and electric window operation, also common for all V8 W116's.
Lift up the carpets and have a smell for evidence of moisture being there, at sometime in the past. Moisture that has been allowed to accumulate under the insulation / sound deadening can cause rust in the floor pan. Have a quick look at the area above the dashboard for cracks. Look closely at the operation of the sun roof and of evidence of rust, especially along the leading edge.
Have a look at the brake pads and discs, and ask when they were last replaced.
Look closely at the condition of the upholstery, especially the driver's seat. Expect to find at least minor cracking of the leather here. Easily hidden with seat covers.
A lot of these checks are standard for any classic car.
Might be a good idea to write up a dot point checklist for yourself, and take that with you. When you have seen a car that you may consider to buy, get a rough estimate of all of the work that might be required and prioritise that work the way you see fit.
There a few of them around when you REALLY go looking. The example that you find in the best condition should also work out in the long run to be the cheapest to buy, in terms of overall cost. Take lots of time and patience. Be choosy. Ask for all receipts that the previous owner has kept. If work has been done and there is no receipt, ask why.
If you get a nice example, it will be the source of a lot of automotive satisfaction.
