I'd suggest a winter beater for those really awful winter days/weeks. (Also for those really awful summer days, for parking on the beach.) Not all winter days and weeks are awful. I bought new snow tires for the BMW last year but never put the winter wheels on the car because the bad weather mostly occurred at the beginning of the season.
I'm guessing that you had planned for a second car, anyway.
https://www.roadandtrack.com/car-cul...inter-beaters/
https://www.caranddriver.com/feature...inter-beaters/
I live near the ocean and our cars get a double whammy -- salt on the roads in the winter, and salt spray at the beach in the summer.(At least the Eurocars don't dissolve as quickly as our Hondas have.)
The E91 is a cult car, for sure, but it won't last forever, no matter what you do. Also, rodents enjoy munching on the electrical wires of BMWs (and Hondas) that aren't driven too much. Apparently both marques use soybean-based electrical wire insulation. Honda even has a cayenne-pepper flavored electrical tape with an official Honda part number to discourage rodents from revisiting.
I wish I could enjoy mine more. It has low mileage, but not by intention -- I telecommute.
The use of a battery tender is encouraged. I routinely plug the car in on Saturday if I don't drive it a lot (winter and summer). The BMW's electrical system is never really asleep so there is a constant battery discharge.
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2008 E91 36K, 6-speed, RWD, Alpine, Terra, Xenons, ZSP, ZCW, ZPP, PDC, CA, alarm, M shifter and M brake handle, Euro rear fog light switch, Euro aspheric mirrors, rear power outlets, Euro split armrest, Lidatek, remote V-1, Cocomats, beach sand, $10 clutch stop mod. 1998 Volvo V70T5 226K, 5-speed, original clutch (third clutch pedal!), aspheric driver's mirror, E-code headlights, IPD sway bar, strut tower brace & skidplate.