How long, on average, does a 70's Ford ranger F150 transmission last?

Thinking about getting this truck. Guy selling it has papers proving he's had work done for the transmission, but that was 15 years ago. Would I need to worry about the transmission being any good because of age? He's asking 2500, says it drives great. No problems under the hood, just cosmetology.

First one lasted 20+ yrs. Any reason second would not?

If it's an automatic, then it's probably a C6, and C6's are basically indestructible

Cosmetic.
Cosmetology is something else.

100,000+ miles.
But rubber seals get brittle after so many years.
The trans might make problems because of seals, not because it's worn out.