This item on mercedes automatic transmissions appears to run for about three hours. Of course nothing beats prior experience when evaluating if a used part should be used again.
I suspect it is not bad value really. If nothing else in a video format you would know what you where getting into before tackling a transmission.
As with most things the first time would be the worse usually. Really could be quite educational as well. Express your concerns to the vendor.
Perhaps he might give you some prior customer refferences to check with. Never hurts to try. Think of the rush telling guys like us what exactly you suspect is wrong with our transmissions six months or so from now. Priceless.
I also read your thread about the jackhammering noise today. It might not hurt to have a mechanic listen for a minute. Just may not be good to drive with depending on what it is. At present I cannot even speculate on the actual cause. The sudden smoking as well hopefully will help diagnose.