Those old fluid coupling automatics were pretty durable. If it's slipping, I would look into low control pressure caused by a plugged filter, or modulator and/or band adjustment. It might have quite a bit of life left in it.

The average shop is going to really mees up the rebuild on one of these, since it takes an old school Mercedes mechanic with the right tools to get one set up right.

For originality sake don't let them exchange your trans. for a rebuilt. Matching numbers mean alot to the serious collector.