I'm considering installing a Goodman LP Gas Furnace:sizing? Ops?

I'm considering installing a Goodman LP Gas furnace, a 95% two stage gas valve with either a two speed or variable speed blower. I have two problems: I'm not certain of the sizing as several contractors have different estimates, and I don't understand how either furnace works, and the contractors can't seem to explain the process.

My house is a double brick, (two layers brick, three inch air gap between, roamed with open cell foam.) Inside area, 2,300 sq. ft., outside, 2,600 sq. ft. Currently using a 140,000 btu inpurt, 120,000 btu output, 80% afue Luxaire, about 12 years old. Furnace completes two cycle per hour at 20 degree outside temp, heat cycle five minutes. At very low temps, c. zero F, furnace runs about half the time. Estimates on two stage systems range from an upper value of 105,000 BTU to 90,000 BTU, I'm assuming this is input BTU. My windows are ten year old Lowe"s "Survivro" vinyl, basement ceiling is insulated. Last year had an infrared scan done, result, house is "substantially" efficient with a couple weak areas where foam apparently didn't get applied.

Again, my questions, what is the proper size furnace in input BTU, and how do the variable two speed blower and variable speed blower models work?

Thanks for you help...

George