My vote is for the 617 also. I have heard too many horror stories on the 6 cyl. heads.
as for slugishness, yes fix the fuel leaks, check injector timing, and change fuel filters if you have not done that yet. Also since this is "new" to him, he might want to check the tank screen.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project