After much searching I've found a clean 97 E300 w/ 121k that the owner will let go for $4500. Here's the catch... Runs rough, knocking, smoking blue and white, glow plug light stays on. Not overheating. No apparent knock in lower part of motor. Owner said his tech says the timing chain has stretched and thinks it has bent a valve. Not sure how he determined this. Will be asking more. Is this possible if it still runs? Do chains stretch that far? What is the worst case scenario for the engine? Is this a bad idea? Will I probably end up opening this engine up and blowing a bunch of $$ then having to rebuild it in the end? What would a total rebuild cost? Will this motor have problems forever unless it is rebuilt from the bottom up from the start? If I could repair and service it for $3-$4 grand and it be a sound vehicle I would probably go for it. Thank you for any help you can give.

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'86 560SEL-108k,Sold Mar. 2000
'95 Landcruiser-113k,Totalled Aug. 2001
'92 Mazda Miata-22k,Sold Apr. 2004
'89 560SEL-31k,Sold Mar. 2005
'95 E300D-121k,Sold Dec. 2005(Mistake)
'95 BMW 740iL-26k($$ Pit),Sold May 2006
'06 Jetta TDI-8k,Sold May 2007(For MSRP!)
'97 E300D-123k,Sold Jan.2009