I would start by looking for a used Bigfoot in my price range. These are molded fiberglass shell, in three sizes (17.5, 21 and 25 foot) originally designed for year-round use. For permanent living alone, I would be inclined toward one of the 21-foot floor plans, although I know a few people who are quite satisfied with the space they have in 16-foot (and even 13-foot) models of other brand off these "eggshell" trailers.
Originally well built, but used, I think would be a better starting point for adaptation to extreme cold than any new low-cost lightweight trailer on the market. Keystone's Zeppelin line was originally one of their ultralights, when I first saw them in 2004, but the product may have changed since. There is a wide range of wall, floor and ceiling thicknesses, products built for the ultralight market may have walls as thin as an inch, less than 3/4 inch of insulation, and effectively no insulation in the floor. Not a good starting point for modification to winter use.
