There is a risk of tearing the oil pan gasket when you remove the rear crank seal carrier. The shop manual casually says, 'Do not damage oil pan gasket. If necessary, replace oil pan gasket.' In most applications the M103 has to be lifted to replace the oil pan gasket.
FWIW, when I was shopping for an indy to rebuild the transmission in my 91 300SE, none said that they recommend replacing an M103 rear crank seal just because the transmission is out.
83 300SD ... $950, see cars forum
91 300SE ... sold