Don't know alot about this transmission in question but Mercedes Trannys are not your run of the mill domestic transmission. But they do share some things in common. Being rebuilt is usally a hard one to figure out. If its a transmission speciality shop and all they do is trans like B&M, etc. they useally have the best warrantys. Mercedes probably has a reman for a half to two thirds what a new one would run. But we are talking upwards of 3000.00 more than likely. I've got some older cars and trucks 1990 chevy pickup, 1986 Olds 88 and I have used with great success, a product called Lucus for automatic transmissions. These have close to 300000 miles on them and I've always serviced the transmissions. It say it is safe in all transmissions and since it want go into gear anyway 8.00 dollars is a safe shot.
Problem with rebuilt transmission you dont know who did the job. If it was a regular transmission shop you probably didn't get any hard parts with the job ie, torque converter etc. Just clutches bands etc. Hope you can figure out the origin of the rebuild/ Call the guy who sold it to you for follow up info and go from there.
1989 420 SEL 105500 miles
1990 Chevy C-1500 290000 miles
1989 Olds Dellta88 276000 miles
1985 Ford Ranger Explorer 173000 miles
Gravely Tractor 8123 1400 hours
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'