This is definitely not normal.
There is a drain plug on the torque converter. You have to turn the engine such that the drain plug is at the very bottom. Only turn the engine clockwise as viewed from the front of the car. I don't miss the days of having to turn an M116/7 engine by hand.
I had similar symptoms on my 78 450SEL. Not long after the delay when shifting into R or D, I would get a delay even while idling in D. I'm at a light, transmission in D, when the light turns green, I have to rev for a few seconds before the tranny engages... and I never took it out of D. Then it would happen even when cruising. I don't know what specifically was wrong because it was simpler and only $800 to replace the transmission than to have it diagnosed and repaired.
I don't mean to scare you, but look out for the dangerous progression if your transmission has the same problems mine had.