Boyd,

Biggest thing with the 4.5 is to keep up on 3,000 mile oil changes or else you'll experience premature wear on the right camshaft. Also, 100K miles is a good point to replace the timing chain, tensioner and guide rails.

As for the other mechanicals, stick to this regimen:

Every 3K miles: Oil and filter change
Every 30K miles: Transmission fluid and filter change, air filter change and fuel filter change
Every 15K miles: Spark plug change (use ONLY Bosch Super plugs)
Every year in spring: Flush brake fluid
Every two years: Flush coolant (use Zerex G-05 or MB type)

The rear seat cushion is common on W116s. The horsehair gets old and deteriorates. As for the locks, that's also a typical issue, you've probably got a vac leak in one of the elements in the doors. They have diaphragms in them that tear when they age. Also could be a broken line in a door jamb.

__________________
Regards,
Aaron