I am thinking about buying a used BMW 540, either sedan or wagon. I have not heard good things about their performance in the snow. The wagon seems like it should be better with its 50-50 weight distribution. Feedback would be appreciated.

Its a rear wheel drive car, it wont perform like a 4wheel drive. I would preffer to drive 5 days in bad snow than to drive a front wheel drive car for 365 days a year. That's the trade off. If you use snow studded tires it wont be so bad.