old gas furnace and passive solar heat

[peachlizard]

Hi everyone, I'm looking for opinions, facts, ideas, etc.... We have an older mobile (70 something model)as a getaway on some lots we own near a lake. It does have an old gas furnace that was apparentily being used before it was repoed. However, the place was roach infested then and although I've only found a very few dead ones, I'm worried with the nastiness of the place the duct work will smell bad if used. And of course we'd have to have the furnace checked out. We have not decided what heat to use this winter, as we're not up there a whole lot because of having to drain pipes and winterize. This place is in Northcentral Arkansas and the front faces south. When we are up there I want to use passive solar as much as possible. We have a small deck along front that is 4' wide x 19' long. We have a sliding glass door, and a regular trailer door that both open out to this deck. The 6' immediately in front of glass doors is covered with a roof.
I want to know which would serve us best for passive solar heat. Enclosing the roofed area of deck, 4' x 6', with plastic to send heat thru the opened sliding glass door (EASIEST), or building one of the passive solar heaters that hang on the outside wall? Eventually I want to make this deck an enclosed porch with windows. I appreciate all your input.
Also for true heat, especially at night, should we mess with the gas furnace, besides the aforementioned worries, I've heard they really use the propane quickly and it had an ancient a/c unit outside that we didn't get. Will the furnace work stand alone, without changes made due to a/c unit being unhooked? Thanks EJ