Purchasing Furnace - 2-Stage Variable vs. Standard

Variable speed AC motor, 2 stage gives more comfort and even heat on low btu heat 1st stage but wont save you more on utilities. 90% is the lowest efficiency on high efficiency units, 94.5 is the high average with 96 being highest. An 80% furnace replaced with a 90% is going to save you 12.5%. But for the same money you could get a higher efficiency unit saveing you 18.5% a 94.5% unit. You stated a crack exchanger, did you see it, because by law im pretty sure they were required to lock your unit down or they are liable if you get sick. So I question that. Is your Co meter digital with Peak Memory. Low lovels are bad , low levels that dont alarm, if you even have a cracked exchanger which is doughtfull yet 40000 a year at least are made sick by Co. You need a load calc or they will likely oversize you , a written one in your hands. Windows and insulation are smart you really should do both and have the calc reflect both ways. Your short cycling may be the thermostat set for to narrow a band. I would say be sure you have a crack and get a 94.5 % unit and upgrade windows. But you are buying before knowing the right and best option and posibly the truth. You should stop the order and do research first after you learn all options then act.