Vacuum line from flame trap on early 850 doesn't go to pass.side of intake manifold.
It goes to the vacuum tree.

I replaced the engine harness on my 1983 240 many years ago. I tried to work from one end to the other as much as that is possible. Do a good wash down before you start. Get the front raised on blocks or jack stands because some of the harness likely will need access from the bottom. Take many photo...

As you have already found out, vacuum hoses are a good start.
Putting aside the obvious (timing belt etc), I would think engine mounts would be next on my list.

Alternator brushes, Thermostat, coil, rotor and cap, brake pads and hoses, brake flush, bulbs if you don't want to worry about them failing while driving.
volvolugnut

Stage one…all the fluids…and brake fluid, 250 ml from each wheel caliper

...I am just going to start replacing things once or twice a month even if they don't need it so over time 30 year old parts will be gone and new stuff will be there... Pick and choose carefully. Replacement part quality has been going down, and you don't want to replace an old but working good qua...