1993 300se timing
 

I recently had a shop attempt to replace the head gasket on my 1993 300se. The car ran perfect going in and is now undriveable . They say the timing advance won't stay in place and it needs to be replaced (the gear). Another shop said it was the harness insulation gone bad from the head removal and that that needs to be replaced first. The car is making a rattling sound when you accelerate, along with no power. After a head gasket replacement, what did they most likely damage to create this mess- the gear, the harness, the computer, all of the above? It is a 3.2l 6cyl. The rattling sound is not preignition, it sounds like a gear, and only on acceleration. The car also idles horribly.
Is there also a timing chain tension issue? I would greatly appreciate any assistance in this mess. Thanks

The shop you took your car to - was it an MB specialty shop?

I noticed the term "timing advance" in your post and right away wondered if the crew you hired has ever been inside an M104 motor?

All cars have some form of timing advance, but I wonder if Chevy short blocks with geared distributors are more what they're used to seeing?