I have the same thoughts about unvented heaters. If you go that route for certain have a CO detector functioning.

As to heating the space, your foundation walls are, in effect, a heat sink. The basement remains at 55 degrees because that is ground temperature. Heat goes to cold, so without anything to insulate those exterior walls it will be difficult to make the space comfortable.

If you have the standard basement windows, cover them with plastic to better seal them. That will at least reduce some draft.

Our basement is finished and when the furnace runs it warms to house temperature. We installed an electric fireplace for auxiliary heat, generally needed spring/fall when the thermostat isn't calling for heat in the house.

Hope this helps.

fnmroberts--What kind of flooring do you have in your basement?

ermachef - Just a vinyl 12 x 12 tile - with a not-slip surface. It is the self-stick type but I put down mastic because I didn't want that hollow sound where there are slight depressions in the concrete. No problems after about 8 years.

Here's a link.

Here is a link that might be useful: Basement