Even though it’s been unusually warm around here, I made the decision to replace the furnace. The old beast was working, but with gas prices so high its ~50% efficiency left a huge hurt in the gas bill.
There were some minor issues, but all in all things went very smoothly. It took the afternoon to remove the old furnace and leave a big empty spot in center of the house.
Then, on New Year’s Eve, I was able wrestle the new furnace in place. I used some concrete backer-board to raise the furnace off the ground and line its height up to the AC coils. The concrete backer-board also allowed me to level out the floor slightly. (Actually, I made it slightly unlevel to keep the water from the coils flowing down to the drain hose.) 
Then I built the return plenum and repaired some of the poorly done vent ducts. To connect the gas, I cheated… I used a flexible gas pipe to get from the valve to the furnace. Easy, cheap… no leaks. Lastly, I connected the new super-fancy computerized thermostat. This alone should cut my heating/cooling cost significantly.
So… installing a furnace is something you can do yourself! Of course, warm winter weather is always good to give you time to do it.
PS. Happy New Year!