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.