It would be best to replace the furnace before it fails

My current furnace is nearly twenty years old and definitely showing its age. I have been reluctant to replace it because the heating system is still operational. Every time I get ready to contact an HVAC company to get a quote on a new furnace, something else goes wrong in my home. Over the last couple of years, I’ve replaced the roof on the house, purchased a new washer and dryer, invested in a new water heater and gotten the septic pumped out. I realize that even though the furnace still runs, it’s costing me a lot of money. I’d spend less on my monthly heating bills with a new, more energy efficient heater. The current gas furnace only achieved an 86% AFUE rating when it was brand new. Over the years, efficiency has steadily diminished and running costs have gradually increased. The furnace has some trouble keeping up with especially cold temperatures. On really cold nights, the house feels chilly. It doesn’t really matter how high I adjust the thermostat because the furnace is no longer putting out as much heat. It runs non stop, consumes a tremendous amount of energy, but can’t combat temperatures below zero. Plus, the furnace now makes a lot of noise. Every time it starts up, there is a screeching sound and a weird rattle. I’m rather concerned that the furnace is going to suddenly quit in the middle of a blizzard. Right when I need it the most, my family will be without heat. This would force me to be in a rush to replace the furnace. I’d need to accept whatever model is available rather than choosing the ideal system for my specific home.
Heater maintenance