I just repalced the front crankshaft seal on M103 engine. I borrowed the tool that locks the flywheel from my friend that owns an independent MB repair shop. See if you can borrow or rent it from someone. It make the job so much easier to both loosen and then retorque the crankshaft bolt. Torque on bolt, with oil on the threads, for M103 is 148 ft lbs then another 90 degrees.

My buddy also said you can hold the crankshaft with a chain wrench on pulley with spare piece of belt on pulley to protect it from damge. I don't think I'd be strong enough to to it that way.