OK, where to begin, where to begin....
The car you are looking at is a '75 model. That means EFI rather than CIS fuel injection. The EFI did not age well and you're always fiddling around with something on it. As Rick said, the W116 is known for rust problems as they age. Thoroughly check the rocker panels (especially around the jack holes), the rear quarter panels at the bottoms, the door threshold panels, and the firewall for rust. Firewall rust is about the most dangerous place it could possibly go. The reason for this is because the front stabilizer bar mounts to the firewall via half-moon brackets and when the tin worm hits there, the mounts can suddenly break away from the firewall at speed causing a bad accident. You must take a screwdriver and poke thoroughly in the front wheelwells at the metal around where the stabilizer bar comes out. If this metal is weak, expect the same for the firewall. And as you can probably figure out, it's a nasty job to replace the firewall! Pull up the carpets and make sure there are no holes in the floorboards and also check the spare tire well.
Getting back to mechanicals, the 4.5 engine is virtually bulletproof but watch for camshaft wear and/or noisy lifters. The cam followers can also wear. Check for oil leaks and also check the tranny for major leaks. This car wouldn't have the automatic climate control yet so you're lucky there. Driver's door window regulators are known for breaking constantly, so if possible, make sure the windows work. It's alot of work to resurrect an old car that has sat idle for long periods of time, but with some $$ and some TLC, I recommend you go for it! But most importantly, watch that coachwork for deterioration!