Most 300Ds aren't worth more than $1000 anymore despite their condition because it's a real hobby car. If you can get it for that price, I would check the trans fluid and sew if it's low or smells burnt. Worst comes to worst you have to put a trans in it or rebuild it. Neither which are not difficult.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k