I had an old furnace, the heat exchanger was starting to go. Yours probably isn't far from that given its age.
I got high efficiency, it was maybe 7 years ago when there were tons of government rebates, so it was almost the same price as mid. The nicest part is the high efficiency brush-less fan unit, as it uses minimal power, but I can run it most of the time. It circulates cool are from the basement in the summer, and keeps the basement warmer in the winter. Without the brush less motor, your energy bills will be high if you do that. So hat was one big benefit with the high efficiency, though maybe the mid ones have that now too.
They did have to run fresh air lines, they will find the best location when you get your estimate. One other thing I got was an electronic damper that only opens when the furnace is on, so cold air doesn't seep in in the winter and hot air in the summer when circ is on. Well worth it.
In the end, becuase I run the fan all the time my energy bills actually came out about even, but the house is more comfortable.
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