Didn't I read somewhere that the metal/heat treat began to improve in 1986 / 1987??
True, the Metal Heat treating was significantly better during that time period and even better for the Series 88 ( 1988 ) Pistols.
What do you think the longevity of each handgun is?
Given proper maintenance of Springs and Lubrication, The Tz75 would probably last 20,000 rounds or so.
The Tristar would easily last 35,000 rounds or so. Note that some Tristar and Canik Pistols have survived NATO testing of about 64,000 rounds.
Why wouldn't you suggest a TZ to be used for self defense?
You could, if you did not shoot it alot. Also, I would Loctite the Slide Barrel Bushing in place, as they often come loose.
There is always the trust factor with using older firearms for Self Defense, yet I have. But I knew their condition well.