IMO you should stay with om60x engines, like you already have. So an SDL or 87 300d.. could actually go om601 if you really wanted. But I would stay 602/603.

The older 300d/300sd's(om617) are just a total different beast that will not share common parts with what you have.

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D