Playing devil's advocate, that's disposable car money and hopefully optimistic on his part. If you enjoy learning new things, go for it. As complicated as a W140 Diesel is, it's no more likely to leave you stranded than a W124 Diesel. Bent rods will get you home after head cracks leave you stranded.

Since the engine runs, you can check ACC function which should give you a better indication of the value of the car.

What's the mileage on it? Maintenance records? Indication of replaced or rebuilt engine? How do you feel about doing a 3.0 block swap down the road?

If you're the type who'll be discouraged the first time the trunk lid doesn't close fully, walk away from any W140.

An S320 is rated at 15/22 mpg, an S350(D) is rated at 19/26. Mine got 25 mpg combined and up to 28 mpg highway.

Sixto
87 300D