I'd sell the SD...poor seats, no heat or AC...seems like its in worse condish than the SEL. The SEL would be second on my list to sell, but it might be difficult due to fuel economy concerns, etc.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)