If you're looking at single speed, consider an internally-geared hub- you get the same advantages of no derailleurs or cassette to clog up with snow, but retain a decent gear range. Sturmey-Archer three speeds are simple and quite robust. More modern hubs are available for a reasonable price with seven or eight gears. My three-speed is set up with the high gear as a flat ground gear, with the middle and low gear used for acceleration, climbing and fighting headwinds. You can always coast downhill if you need to.