I've heard of the non-turbo engines, like the old 220/240D engines of going half a million miles. These turbo 300's seem to be more around the 300k as an average. Of course depends alot on how well it was taken care of.
Maybe someone else has more insight on this.

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)