Aside from how much you can/want to spend, I think it comes down to what you want out of the Z. if you want to keep driving it and enjoy when you take it out, and I say fix what you can see, and the small things that pop out, but with the expectation that rust repairs could expotentially grow. This will generally get the car back to you quicker, and should be cheaper. Also, un-restored Z has hidden rust.
Exhibit A: https://www.viczcar.com/forums/topic/19666-project-factory-fresh/
Exhibit B: https://www.viczcar.com/forums/topic/30840-redrbzed-minor-rust-repair/
Exhibit C: https://www.viczcar.com/forums/topic/25010-micheals-bent-bus/
If you want to go back to bare metal on the whole car, i.e. sand blast and rotisserie resto, then you most like be spending well into $100k, that's assuming you want to do everything right, because why would you not if you have gone to this effort. Just my opinion. And like you said, you probably wont drive it as much, and be too worried to park it anywhere. But you also said you have another Z, so this might be worth doing.....
In my current situation, and probably for a long time, I would fix the known issues. My Z cost me less than most running 2-seaters at the time and in the current market, maybe because of its wonderful V8 . I purchased it 12 months ago. I am not overly worried to leave it out of my sight for a while (not hours at a shopping centre though). I dont plan on driving it in the rain (need to upgrade the wipers), but it has been in the rain and that doesnt really phase me. It may even see a track day or two.