Steering rack,rive belts,seat bolster.
I have owned 4 of these cars,and they all suck.Don't listen to anyone else,I use to work @ a shop that worked on 300zx exclusively and they stink.The engine will blow for no reason,compression ratio will die for no reason.Just a nightmare car ingeneral.That statement has nothing to do Nissan line of cars as a whole I have owned a 240sx and beat the mess out of it with no problem.Bu the 300zx is made from twigs and rubberbands.
Those things sound like regular wear and tear for a model year 1989.
Plus you really sound like you work in a 300zx exclusive shop when you say, "they blow cyl for no reason", "they lose compression for no reason". There is a reason and its not the car's fault, usually, but you sir should sit down and shut up.
I don't understand why you've bought four of these cars if you don't like them.
You must not be much of a Z mechanic since you did such a poor job of checking out the vehicle before you purchased it. Engines don't blow for no reason.
The Z31 chassis is extremely reliable, however it is 16 year old car and without proper maintenance-like any other vehicle-they'll show their age.