David is right. The cost of switching the engine would more than eat up any savings a diesel might give you. Its alot more involved than just putting in a new engine. Your transmission might not mate to the new engine, the exhaust probably wouldn't mate right either, then there is new ignition, fuel delivery, engine controls, and on and on. Like david says, you'd probably have to drive for 15-20 years on the new engine to break even on the gas.
1995 E420 - retired
1986 420SEL - retired