I think we might have a 2 pager going here. I'm cheap and I do my own work. (My car was screwed up too many times at the dealer.) That said, I would go for a few hundred $ used one IF the wrecker could give support to the tranny being in very good working order with very few miles on it sinced installed. You can usually get a 1 year guarantee. But, on my SL, I would be very hesitant to go through all that effort with the standing risk that in a year or two or three (since we often don't drive them much and when we do we treat them like old ladies) You aren't going to find out you made a mistake until its too late or you'd hafve to pay the guy to do it all over again. I would much rather tear the original apart and replace all the wearable stuff that leads to break downs or poor shifting: clutches, bearings shims, springs with obvious wear, switches, modulator...hmm what else?
Or if I'm not doing the work, no brainer find a wrecker who rebuilds, or an exchange rebuilder who has one. If they have one chances are you aren't their only customer for them.
Either way, this is a heavy girl with enough RPMs, torque, and HP to 'get scratch in 2nd'. I don't want a weak tranny under my feet that can't take the fun I'm ready to ask it to deliver. When I downship to go up a hill or pass a 911T, I don't want any witnesses to think I'm driving a girly car.
JMO Put in a known good tranny.