I'm thinking of turning my crappy box-store mtb into a fixed single speed (no freewheel) with front disc brakes. I plan on replacing the rear sprockets with ss specific ones (bigger teeth) and getting a new crankset. My ride is mostly flat too, so a 3:1 ratio would be decent, I think, especially in the winter. Most people with this setup will get a 42 tooth front and 14 tooth rear, which are common enough to be found in most shops.
I need to gain some more knowledge before diving in, but I think it will be a great winter bike if I get it done.