OK, guys, here's the scoop. Yes you can buy the LCD unit separately from the cluster. The dealer will happily sell it to you for some obscene amount like $300. I can dig up the part number if anyone REALLY wants it. The better method is to buy a used one at a salvage yard. Most of the major MB yards (Silver Star, PG Auto, etc) will sell them for ~$75. I just bought one from PGA for $75+s/h.
Pinout: The older models had 4 pins only and no connection to the Hall sensor. From about 1990 up, MB improved the design to eliminate false readings at low speeds. This unit has 5 pins and the extra pin is for the Hall effect sensor. You do not need to connect the 5th pin, it will act just like your old one did (you know, showing 120F at idle in summer when it's only 90F outside.) As to how to wire the 5-pin display to a 4-wire car, follow the schematics in this PDF file:
http://www.meimann.com/docs/mercedes/LCD_temp_testing.pdf
Can you add the signal from the Hall effect sensor to a car that originally came with a 4-wire display? I think so. I researched the parts used for the old & new cars, and my 1987 300D has the same Hall effect sender on the back of the speedo as the late 124's up to 1995. It looks like a matter of tapping into the Hall wiring and running a feed to the extra pin. I have NOT attempted this yet, but I plan to in the near future.
HTH,