This sounds crazy but you might want to check with the dealer parts department. If you have a relationship with a dealer parts manager or can build one, they can be priced very fairly. (i.e. come in near the end of the month when they want to hit the sales quota and politely ask for the "best price" to beat the dealer across town)

My C32 had a factory trans installed a few years ago and parts cost was $3,500 or less- and that's at full retail price through the service department. On a wholesale account, the transmission and converter could be had for under $3,000 at the dealer, I reckon. And that includes a nice, MB-backed warranty.

eBay rebuilds--- that seems like a real throw of the dice. eBay parts have their place; I'm the first one to recommend getting used stuff, or sketchy eBay stuff to save a buck, but a transmission is a lot of work to have to do it twice. I'm only saying that because I've been there before.