I am doing some retirement planning. I want to estimate the cost and
lifetime of my HVAC system.

I have central gas heat and ac. The is 2 story and about 2300 feet.
The house was built in 1991.

Also, I would like to do some planning for what to expect. How long
do they last?
I see from some other posts that they may want to replace all the duct
work. What should I look at to prepare to replace these systems. Can
I just replace the AC unit or gas furnace, only the part that fails?
Should I do that?

The upstairs unit is rarely used, only when we have company.

The downstairs outside AC unit is a TRANE XE 800 TTJ730A100B0.
Upstairs outside is TRANE XE 800 TTJ724A100B0. My web searchs have
not even turned up the tonnage of these.