Timing chain, guides, and tensioners done? Have you looked for seepage from the head gaskets, rear man seal and tranny pump? Other than owning one 500SL for the last 10 years, I've sold 2 low mileage original cars in the condition with the documentation you describe. One with a beautiful Gray(same as yours)/Red color combo and 82k for $13500, the other Manganese Brown/Tan with 75k for $10750. The Brown one sold last November, and was shipped back to Germany and resold there. I paid under $8000 for both of them, and the only needed minor reconditioning to be made "retail ready". I certainly left a few grand on the table with the brown one, but I wanted a quick sell and thought the color would hurt it a little.
Gray selling live on video:
Blacktop Nationals Auction of 1984 Mercedes 500SL - YouTube
Brown bat ad:
http://bringatrailer.com/2012/11/18/bat-exclusive-euro-spec-1984-mercedes-benz-500sl/
I chuckled at the investment idea. Mine is driven regularly as a spare toy, maintained to a top notch standard, but manages to find about $2000 a year in unscheduled repairs. If it weren't for the huge sentimental attachment and how good it looks on me I would have a hard time justifying holding on to it.
I never even bothered listing either here as there's not many buyers on this forum, and most of them want cheap stuff or diesels. I sold my brown one within 24 hours on bringatrailer, and the gray one at a classic car auction. I still have the German flipper's contact information if you're interested. It was a very easy transaction. Its probably a 25k euro car over there, so there's probably some meat on the bone left for him after shipping it back.
There is no rhyme or reason to the prices R107s are selling for, so I won't even begin to comment on what is fair for yours. They are all over the place with no predictable pattern. The most glaring example of a ridiculous selling price was this car at Barrett-Jackson in 2011.
1982 MERCEDES-BENZ 380SL Lot 1527 | Barrett-Jackson Auction Company
I was floored, since I could have found a dozen or more 380SLs in that condition for sale with a 5 minute internet search for a third of the sale price.