You forgot Rice, you can eat it later.
I don't put any weight in the trunk aside from my toolkit. (now I have a baby stroller in there all the time... grrr...)
Honestly the main problem you'll have with the M3 might be the fat tires. Also the isolation from the car, modern cars have less feel and that can be dangerous when you place too much faith in it.
My car has no gizmos, my brain handles traction control and all associated systems, and it's served me well so far.
Get good winter tires, add some weight if you feel it helps, keep a kit in the trunk for emergencies, and go into an empty parking lot for a few minutes and slide around a bit to get a feel for the car in the snow. Don't do it too long or make too much noise or the cops might think you're a goof and give you a ticket.
Best of luck and enjoy the winter!