Wrong temperatures, sorry.
It was 10 below that night, not 20, the allowed temperature raise is 60 degrees, the stable inside temperature would be 50 degrees.
I STILL wouldn’t buy the house.
Unless I’m quite confused, the “temperature rise” is the difference between the intake and outlet temperatures. Take your house at 10 below and turn on the furnace. Intake air is 10 below, outlet temp is 50. After a while the intake air begins to rise in temperature and so does the outlet temp. A few hours later the ambient air temperature inside stabilizes at a nice 70 and the thermostat shuts down.
The only way you would have a stable 50 at the conditions you mention is if you were stupid enough to draw all your intake air directly from the outside.
Your results may vary depending on size and insulation of structure and size of furnace.