they didn't drain the convertor, if you don't the fluid will usually get contaminated right away. total drain and refill with filter is around 8 quarts of fluid, but you are well off getting the correct tool for checking the fluid as to not over fill. in a shop environment the way i do it is i add initially 3 quarts start the car then add 3 more while it is running, go for a drive to get the trans up to temp, mercedes says 80 degrees celcius, then check it and add accordingly, at 80 it needs to be at the top of the upper mark on the tool. to get the temp i use the factory mercedes diagnosis computer, but i would think you could use an infra red thermometer on the pan. also since they usually leak, it is a good idea to replace the data plug on the side of the transmission when you do the service

buy:
one filter 140-277-00-95
one gasket 140-271-00-80
one dataplug 203-540-00-53
one magnet 220-271-00-98
8qt mercedes atf
2 seals one for each drain plug 007603-010100