Ah, I think I'm beginning to see the light! So, if we didn't have an air cleaner installed with the furnace, we then would have to change the filters monthly? Is that it?
If you decide to run the fan continuously, the filter will have to be changed more frequently. The 5" filters are ideal (only need to be changed every 6-12 months); however, changing a 1" filter once a month isn't much to ask when it comes to maintaining an expensive piece of equipment.
Basic 1" filters (not allergen reduction) range from $5-10 a piece. Fiberglass filters are also available for <$1 but they do a very poor job.
Ecm blowers (variable speed) consume far less energy than their multi-speed counterparts, especially on low.
Since changing the gas valve has bought you some time, consider reducing heat loss if practical/economical prior to replacing the furnace. 80k (76k output) is still a lot of heat for 1400 sq ft; you will probably have high heating bills in spite of running a 90%+ eff unit.