The legendary 3.0 liter 5 cyl. turbo diesel that Steve mentioned was discontinued in 1985. There are 2 big bodied cars that had this engine, the W116 300SD model years 1978,'79 and '80. Then MB brought out the W126 chassis and continued the 300SD until 1985. In 1987 they came out with the 300SDL, which was a 3.0 straight six engine. This engine proved to not be a durable as the 617series of 5 cyl. turbo diesel engines. I had a 1980 300SD (W116) that had 280,000 miles on it. My brother now has it. Fuel economy in that car was about 28 mpg. I would imagine that is better than your 450SEL. As for which big bodied diesel is better that's a tough one. For that classic Mercedes look, you would have to go with a W116. Unfortunately these cars were known to rust bad. The W126 chassis didn't really have a rust problem, but they had plastic bumper covers and didn't have that heavy solid feel like the W116 did, so I have read. You will have a hard time finding a W116 car, simply due to their age. The "newest" one is now 22 years old. Whereas you can find a 17 yr old W126 300SD.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section