The delay and the cost of the hitch system is an issue. I've used trunk mounts, including Allens, on a variety of cars since 1970 and have never had one that did not cause some paint damage-sorry. You also need good spots to hitch the hooks to and I don't know what the M3 has to offer.
Right now, I put my bike in the back seat. Obviously, with more bikes and more people that does not work. I have not decided to get a hitch yet. If we need to also carry my wife's bike, we take the Subaru Ascent with the hitch and rack.
Good luck.