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This is coming from a service tech point of view...
The manufacturers state a expected lifespan of a furnace to be 15 years.
If the furnace is currently 14 years old...and finances permit...I would strongly suggest the entire system.
If you install a 18 seer ac unit with a 14 year olf furnace you will lose 3-4 seer. The seer is indicative of the match set...if you vary from one or the other..the seer drops.
My own opinion is to replace the set and keep them as a set and not mismatch the seers.