I got rid of my w126 2.5 years ago because it had developed an engine problem that would have cost too much to fix, and I still miss that car to this day. The E420 is a great car, but lacks the presence of the 420SEL. If there is nothing wrong with your car I would keep it. Also, it is not always a good idea to do this kind of personal transactions with someone you work under. If you sell it and he has a big repair to pay for within a year he may hold it against you.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired