The best test of the wires is age -- if they are more than four years old or so, or if you have put considerable milage on them, replace them with good wires. Can make a huge difference. So can chaning cap and rotor, as this car doesn't have a catalytic converter, I don't think, and uses relatively low ignition voltage.
You can "test" the wires by opening the hood on a dampish evening or nite without any lights on and the engine running -- bad wires will have sparks jumping around on them. Plugs go bad with age, too, so don't ignore them, either. Ditto for the points. I'd not dig into the injetion system until I had done a full tuneup and made sure the ignition system was in good shape. Next I would check for vacuum leaks, then to into the injection stuff.
The manifold pressure sensor is a round canister-like object on the firewall or on the fender connected to the intake manifold with a large hose (1/2" or so). If that hose is leaking, the car will run poorly. Usually rich, so you would have black soot in the exhaust. Usually, if it is leaking a lot, or the sensor is bad, the car won't start and run at all.
I'm pretty sure you will need someone with experience in the D-Jetronic system to get this working properly -- it hasn't been used by MB since 1979, so you may need to find an older indie mechanic who remembers it. It can be kinda flakey.
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!