Look for early '80s Japanese (or Trek 400 level or the Japanese built Schwinns of those years. There were many bikes made than that serve very well, with horizontal dropouts, fender eyes and many, especially of the earlier '80s, built for 27" wheels and therefor have tons of clearance for big winter tires and fenders. Many of those frames had very decent riding characteristics. You won't rave over them, but they will serve very well.

I ride what I speak. My workhorse fix gear started as a Peugeot UO-8 in 1976. That frame was replaced by a sport Japanese built Schwinn (fun ride!), then a Sekine, then a Miyata 610 (43,000K), then a 400 level '83 Trek (currently at 27,000k). These frames were picked up after the previous was crashed and I got what I could find quickly. Generally quite used from bike shops and with little documentation so I never knew exactly what they were, nor did I care. In total, that "bike" - I consider it to be one continuous bicycle, just one that has seen everything including the frame replaced at least five times - has been ridden in excess of 110,000k in all kinds of weather, much if it in the dark.

Another real plus of sticking to the bikes I am suggesting is that the Japanese did us all a favor by standardizing frames and parts. Over those years, headsets, bottom brackets, seatposts and tubing outside diameters varied little, making swapping the parts out child's play.

Ben