How much would you trust an older (late 80's) Carrier 3-ton? It's been serviced and cared for quite well.
A lot depends on the environment your in.
If it does not leak... that will help.
As far as trust????
That's a lot of run time.
Eventually something will break.
The Carriers I have that are that old cool critical equipment.
They have a manufacture date of 10/88.
The AHU is a commercial unit but the condenser/compressor is a residential type.
1 AHU with 3 condensers, staged as needed.
I replaced the compressor on the first stage last year.
1st stage will have the most run time.... runs most the time.
Staged as 1st stage 2ton, 2nd stage 3 ton, 3rd stage 5 ton for a total of 10 ton max when all 3 stages are running.
If it was my choice they would have been replaced twice by now.
They run way more than yours would.
I live in SE TX. Not only is it hot.
We cool equipment 24X7X365.
I live in a costal area full of petrochemical plants.
Everything rots here.
So I guess that bodes well for you.
Your best bet is to save up and be prepared.
Only you can decide if its worth changing it out for more efficiency.