The quality of the parts people varies significantly from dealer to dealer. Some of the guys (and girls) are very, very good - some of them are not so hot. The marketplace will decide who wins, buy things from the good guys and avoid the bad. To me its worth spending a few extra $$ if the person behind the counter is good - you are building a relationship and the parts guys can be incredibly helpful at times. That said I would not pay $120 for a cap and rotor for a 84 280....
I paid $418 for the harness from the MB dealer - it was a wholesale price, I don't have the list price. Part number was 124-440-19-06. I think fastlane has them for $475 or so.
Install is prety straightforward. Layout the new harness on the engine and see where all the wires go - then figure out what the things you have to disconnect to make it all happen. Don't be intiminated by the number of wires and connectors - its not as bad as it looks when you take the new harness out of the box. Most of the connections go back to the main engine ECU so I did disconnect the battery before starting - I think this is recommended in the MB service manual but I can't remember.
To inspect the old harness cut a small slit in the plastic covering and take a look at the wires. Be careful not to cut the wires under the plastic sheathing. If the wires are bad you will see a powder that is the old insulation and lengths of bare corroded wires inside the harness sheath. Its pretty obvious. You should also check the wires that go to the EA motor that controls the throttle valve. My EA motor had been replaced prior to my ownership and the wires were fine but I have also heard these wires also can fail.
This is not a tough job - I would rate it about a 3-4 on a scale of 10.