Good Coal Add on Furnace on a Budget

In general a unit that burns wood and coal is not going to do either well. also I fully understand the budget aspect of your thought process but let me mention a few things to consider. Most any coal burner you buy will be able to be moved with you if you move. While a unit that costs a few thousand may seem expensive, it's going to do the job and pay for itself quickly. You should be able to tell the propane man to take a hike. I'm sure others will chime in but with your size home, I'd look for a small stoker boiler and put a water to air heat exchanger in your ductwork. A keystoker K2 would be perfect. Weigh all your options before you jump.