I think a rust free Benz is always worth $1500 if not trashed otherwise and the repairs needed are not excessive to get back up to par. But I'm not a expert on this subject. Daw_two, Pierre, or Tyler would prob be who I asked. Around here you can't even get a Camry/Accord in good condition for that, and your TD will be a appreciating asset if you keep it up!
I think you did good!

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2006 Jetta TDI DSG 320k miles
1997 Ford F150 325k miles 4.2L V6 "Work Truck"
2008 Tundra 225k miles 5.7L
1982 240D.....sold
1984 300D...Totaled OUCH!
1985 300D Turbo 222k miles "Dos" sold to 79Mercy
1986 300SDL 98K miles "The Beater"....sold
1987 190E 2.3 16v Euro spec 115K miles....sold