I would look for a good used one in the junkyard. Idk about up there but a used trans that has relatively low miles can be had for a few hundred bucks, and another few hundred to have it installed.

With a 91 300se it's likely that many, many w124's will have the trans you need. There is a part # on the trans you will want to match up.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2