"The current furnace is 125,000
BTU 60% efficiency, the new one will be 110,000 and more efficient
(80%). "

I wouldn't spend $500 to repair a 60% efficiency furnace. I didn't
even know they went that low 18 years ago. You didn't specify the fuel
but if you intend on staying in the house for more than a few years and
it's anywhere with cold winters, I'd go with a 90% or better furnace.