Just wanted to add my two-cents.
Denrac posted two links above - neither one works, but if the link to mercedesdismantlers.com is for the company in the San Fernando Valley, that's where I got my transmission. Upon installation it ran great! Lasted about 3 months before it nearly left me stranded. I live close, so drove my car there and they replaced the unit without question. Great! Four months later it was shifting poorly again, they replaced it again - no good. I took it back a couple of times and they never did get it right. They didn't seem as willing to make it right the last time. Complained about my car taking up space on their lift - as if it were my fault! It's still not right.
As far as bulletproof goes, seems like the OEM units were pretty darn strong. Don't forget, most of our 300Ds and TDs are now 20 years old with many, many miles. My wife's 1999 Taurus tranny began slipping at 35K. When I can sink another $2800 into my car, I think I'll get the factory rebuilt unit unless another member has had a GREAT experience with something else.
BTW - My car's a daily driver and I think nothing of pushing it to 85 mph and beyond (when safe and prudent).

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Dave
1995 C280
1999 Triumph Daytona 955I - my speed fix.
1982 300TD - Gone, but not forgotten.