A two stage furnace will be more comfortable that a single stage furnace. But if you've been happy with the comfort of your existing furnace then it may not be necessary. The different brands are all pretty much the same in terms of reliability. Go with the installer you think will do the best quality job with the installation.

The Lennox is an 80% efficient furnace. What were the others? Just make sure you are comparing apples to apples.

That price seems high for a 2-stage 80% efficiency furnace. What is your location and there a lot of work to be done to remove the 40 year old furnace? Is the furnace part of a split system with AC?

You need to provide model numbers if you are looking to compare furnaces. I think a 2-stage furnace is worth the extra money. It is quieter, few blasts of air, and it will run a little more efficiently. However if you don't have the funds a single stage can work well if it is sized and installed properly.

If you're in a relatively mild winter weather area, a one stage 80% furnace should be adequate. I'd save your money.

Bryant, Trane and Lennox are all reasonably good brands. As some say, the name on the HVAC technician's shirt is more important than the name on the equipment. Meaning, the quality, honesty and skill of the installation and the HVAC contractor is more important than the equipment they use.