My dad has been in the business for over 40 years, still doing quotes for residential now.
I put a new furnace in two years ago. Things I learned...
Most people/installers over size a furnace thinking bigger is better. I dropped to a 66k btu from a 80k btu and went up to a 95 efficient from 80. Runs slightly longer, but house feels more evenly heated. Bills went down about 70 dollars a month avg. Winter 1600 ft2 no basement. Ive got a Lennox.
Price your replacement parts and check availability (even with warranty) to see how long you could be without heat.
Just how much will you save over X years and will it cover the extra cost to go with an ultra high efficiency unit.
Many houses have bad duct design and flow, new or old. If the ducts can't physically handle the flow of air, your new furnace will not perform. Definitely check for closed off vents, crushed returns, damaged ducts.
My dad actually installed an 90 efficient in his new house, said everything was so well insulated it didnt make sense to spend any more. He pays under 100 for heating in the winter, 1200 ft2, no basement. 6" walls, insulated...