80 should be fine...keep in mind, you will need a two stage thermostat that controls the staging, not a cheap thermostat that works off a timer...if you don’t have filter media cabinet,,now would be the time to add one...

Good Luck!

All things being perfect, youre producing 88,000 BTUs.

The right way to determine the size is to do a load calculation. Most HVAC installers don't want to do this and will install the same size equipment with the belief the original equipment was the correct size. The result is over sized equipment which costs more money and does not work well.

A 80K BTU furnace with 96% efficiency will produce about 76K BTUs. That should be sufficient for a 2400 sq. foot house in Chicago that has a good level of level insulation and windows.

A load calculation is indeed the right way. They're required in my state when applying for a permit for equipment replacement or of course initial installations for new construction. Can a calc be fudged or shortcut - of course. A real calc for an average home should require no more than 60-90 minutes in total. Laser tape measures make the work go fast. They measure ceiling heights, walls and windows and the orientation of all of them, assess construction type and quality and insulation, punch it in to a tablet and that's about it.

Hacks will dispute the need if not required. "I know this neighborhood and have been in the business here for 20 years". Not all work is permitted. If the bidding contractor offers any baloney like this, call others. There are knowledgeable and honest folks in the business who try to do the best possible job using up to date techniques. Saying "if what's being replaced worked before, then that's the right size" is also baloney.

What Size Furnace Do I Need?

https://www.air-conditioning-heating.org/

The basic principles the furnace sizing calculator is based on are:

Larger homes need more BTU than smaller homes (directly proportional to square footage).

Homes in colder climates (Region 5) need to generate more heat than holmes in warmer climates