sptt, strongly recommend that you have the tech who will be doing the engine swap talk directly to the seller of the 66k mile engine to verify 1) what peripherals, if any, come with the used engine, 2) that the used engine is a duplicate of your existing engine so that wiring harnesses, linkages, etc. match exactly, and 3) what specifically is done to qualify the used engine as "tested".

As for the transmission rebuild get a firm quote beforehand so you know the cost up front, and avoid any surprises. Also, confirm the tech at your shop has experience rebuilding MB transmissions and offers a warranty on his work.

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Fred Hoelzle