I would look for the latest model you can afford, either a 1996 or 1997. Definitely want one with good servicing history and updated wiring harness. Some say that the W140 was cheapened over time. I know they tried to lighten the look over the car in the later years but I kinda like the early styling. Later models will probably have fewer kinkgs to work out though. Buying one is relatively easy, maintaining one is another story. I think this is why W140 prices are so soft, simply because owners can't afford to keep everything working as it was intended to. The evaporator replacement and wiring harness costs alone can easily total more than what the car is worth.

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1989 300CE