How many miles are on the C230? I would keep the C230 if it is working well. Those M111s are a very sturdy engine. An older Mercedes can be a handful to keep up if you are not careful. I would say only 1 out of every 4 might be worthy of purchasing. The rest are often neglected needing thousands of dollars worth of mechanical and cosmetic repairs that have been let go by previous owners. That being said, a well sorted '91 or '92 300E can be a good buy. Check for all servicing records to make sure fluids have been changed at timely intervals. Headgasket and tranny will eventually need to be done as others have stated once the mileage reaches 150k. I would pay 3-4k for a well sorted example in good condition mechanically and cosmetically.

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1989 300CE