HVAC ball park cost estimates for retirement planning

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Why does knowing the size of both systems bother you?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Just suprising that I could not easily find this info on the web.
Anyway, I found the estimates I need elsewhere.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.