Dude...do yourself a favor and upgrade to LP. You could even have a backup electric plenum heater as a safeguard if the propane runs out or something. Other than venting, a HVAC company can rip out the old unit and get a new one installed in a day. And if you don't have an LP tank yet, a company can still place one and run the gas line above ground now, then bury it in the spring.
This isn't true with current propane and electric prices.
Currently with the off peak electric prices, it would be cheaper to use electric then propane.
1 Gallon of Propane = 27 kWh (Kilowatt Hours) of electricity - This means that one gallon of propane contains the same amount of usable energy as 27 Kilowatt Hours. Or we can say that 27 kWh equals approximately 91,500 BTU.
I have both in my house. I use LP primarly, but have the electric plenum heater as back up source. My current off peak price is $0.052/ KW. Which means that propane prices have to be below $1.40 in order for propane to be cheaper. (27KW x .052 = $1.40 ) Last time I checked propane prices were at $1.59/gal from MVP in Mandan.