I personally wouldn't swap a heat exchanger on a 20 year old unit if it were mine, especially with labor costs that would come in your situation and whatever else could break after. Did you get a year home warranty? A heat exchanger crack would get you a new furnace for free (80% since that's what you have).
If you wanted to go high efficiency, there would have to be a way to run new pvc pipe out the side of your house for exhaust and intake air, would this be possible with it being finished?
Through my company, the difference between 80% and high efficiency is around $1000, which depending on how high efficient, that price can raise. I personally wouldn't go super crazy with variable speed motors or anything like that. We are extremely competitive with pricing and can get you a hell of a deal on Lennox equipment. For us to come give a quote is free.
Feel free to call me with questions. 740-506-0994