As long as you have good, sensitive carbon monoxide detectors, and the furnaces are in good shape and there's no cracks in the heat exchangers and you're not feeling woozy or seeing and hearing imaginary things, you can get decades out of a natural gas heater. Papa Bear's price estimates are pretty accurate currently, too.
As far as the heating and cooling issues at the far end, it sounds like the system isn't well balanced. This said, most heater/air exchanger circuit boards have jumpers or micro switches that adjust the fan speed for the cooling and heating, and a lot of installers forget to monkey with those settings. Since its the time of year to do fall maintenance on the heater, maybe pull up the manuals for your upstairs heater and while the tech is there, explain the problem and ask if the fan speeds for both the cooling and heating can run a little faster. Either that, or see if the evap coil fins can be thoroughly cleaned, they might be clogged a bit cutting back airflow.