I see that you do not want to change the head gasket on your current leaking engine
. I just took off the head of my 1990 300SL-24 today to get the engine to keep the oil where it belongs.
I think that changing the current engine to the M119 engine involves a lot more as one might think...The newer euro MB models do not want to run at all as soon as the engine and...lets say...dash cluster are separated. The data is not only saved in the computers of the car, but is stored in parts like the dash cluster as well.
I think that the 500 suspension is different as the 300 as well and I think that your current transmission will not fit the 500 engine (different flanges).
good luck & greetingz,