Check turbodiesel's thread "My 300SDL on eBay" if you're worried about the engine.... his has 321,000+ on it so another 60k on that one should be nothing if it has been cared for. That's what you have to figure out though....if it's been properly maintained, if it hasn't it could turn into a money pit trying to get it into shape again. $4000 seems reasonable to me if it's in good shape. Shouldn't have any problem switching to a synthetic oil as long as it doesn't have any leaks already.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k