Injector seals will leak, so will manifold gaskets once in a while, ditto for brake boosters (engine will die when you step on the brakes is one symptom), cracked vac lines, bad rubber connectors.
The worst offenders are bad rubber hose bits, where the plastic lines are connected to the manifold, etc. The rubber goes, either into glassy hard stuff or mush, and both leak.
Door lock systems aren't a problem on later models (W126 and later) -- there is a vac pump either in the trunk (W126) or under the rear seat (other models) that runs the door locks.
Vac servos for the climate control can leak, so can some engine controls.
Best way to check for leaks on the engine itself is carb or brake parts cleaner -- spray suspected areas and listen for a change in idle speed. If it goes up, there is a leak. Be careful, the exhaust manifold can ignite brake parts cleaner!
For the vac system, use a hand vac pump connected at the manifold tap. If the system leaks, you need to find the bad part and repair. Some systems (tranny shift control for instance) are calibrated leaks, so be careful.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!