Hey, I live in MN too. The most important thing I can think of is rust, or lack of. I think you should seriously consider looking in the south or out west for a car. You should be able to find any of the previously recommended ones on ebay or in the auto trader online for $1,500 up.
I think the 300d will be the easiest to find, wagons are less common and therefore more expensive, and 240s seem not so common either, but they are cheaper. From what I have gleaned here, the 240s are simpler mechanically, although only 4 cylinders makes them fairly slow. The heater on the 300s runs off the vacuum system, if you get a leak in the system you will only have defrost for heat, with a 240 I think it is mechanical and therefore less prone to failing.
If your car does not come with a block heater you will want to put something in it for winter. Block plugs are very hard to install - not too much room and lots of leverage needed, but you can also stick on an oil pan heater in about 25 minutes.
As long as you have good tires you should not have much problem in the snow, these are heavy cars and go pretty well in the snow. For supreme traction get some Nokian snow tires, about $80 each. good luck, Andy

__________________
andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles