It all comes down to rust and visual quality. With the car up on a lift, check it VERY carefully for rust. While it's there, use a digital camera to take lots of pictures. They will help you get top dollar, if it's rust free. They will also explain the condition rather succinctly if there are problems to address.
The visual quality covers things like the luster of the paint, quality of the interior wood and vinyl (or leather). mechanical things, like does the engine run or are the wheels firmly attached are relatively easy to fix compared to the rust, paint and interior.
The fuel injection system is a work of art (by a mad man
, and if you need to work on that, you'll need plenty of help from us.
If your car passes inspection with flying colours, but is ugly, consider it to be worth 1,200$ to 1,500$. If it's pretty and it passes inspection, you can almost double that. If it's really pretty AND rust free AND passes inspection, you can almost double it again. Very few of these cars are in that kind of condition.
Of course, if it's ugly and doesn't pass inspection it can sell for anything from 300$ to 1,000$ depending on how foolish the buyer is. I paid 2,100$ for my very first benz, a 69 model 230 that was very rusty down below, but very pretty up top. Also, it didn't pass inspection because of emissions. I was totally suckered. There are dumb people that do that from time to time.
If you don't already have it, visit ebay and invest in the 108-109-111-113 service manual. Expect to pay about 40$ for it. A search for "(mercedes,benz) (108,111,113) (manual,service)" should turn up at least 2 copies. Just be sure to leave off the quotes. Alternatively, you can check with fastlane (this website), or call mercedes directly at 1-800-for-merc.
There is a separate book for the M117 4.5 engine.
-CTH