Reasons to consider a new unit are, VS DC motors, 2 stage heat, the possibiltity you are oversized, the instalation of a superior larger size air filter, electric and gas savings in heat and cooling mode, future repairs, and utility costs are going up .
VS or variable speed can run on a Humidistat Thermostat offered by Carrier and Lennox and maybe others, that can be set to run at a very low fan speed to remove double the humidity on days when cooling is not the real issue but humidity is, this feature could lead to you using the AC much more than 10 days a year to lower just humidity with minimal cooling if that is an issue in your area. In heating mode you could run on low-fan, low fire for more even heat, and air circulation through the air filter. VS DC can take 2/3 to 1/2 the electric power of non VS DC for running the fan normaly and even less on low, 1/4 the power.
Your old units blower fan is probably much less efficient than any new unit and you will have electric savings in winter as well, but more with VS DC. VS DC are more prone to failure so a 10 yr warranty, normaly offered as an option would be necessary.
Get a load calculation in writing , from who ever you deal with, if your heat and AC are oversized now proper sizing will give better comfort winter and summer, especialy summer in humidity removal. To large an AC and you cool but remain clamy, to much Btu heat and a to big a blower and you heat to fast and uneven.
Humid areas realy need good humidity removal.
There are many good air filters out there that were not avalaible years ago, that not only filter better on first pass air but last longer due to their total size, so instead of monthly or so replacement it is yearly.
Electric and gas is not going down in price, with the proper unit sized and installed by a Pro new high effeciency equipment, and the deterioration your unit has you could save 50%-60% ++ on electric and gas in winter and 50- 80% ++ electric in summer on AC, and get more overall comfort and control in living.
There are so many comfort options offered today that did not exist years ago. At least look into them your options are endless.