In my opinion that's a tough call. If you want to "keep it forever" then strip it all the way down, fix the rust and start fresh. Zs have lots of rust hiding under undercoating, sound insulation, etc that are going to just keep rusting unless they are dealt with. On mine I blasted the bottom, engine bay and frame horns behind the fenders. Amazing how much shows up after you blast it. You wouldn't want to do all this work to a car, assuming you're going all out, to later have to pull it all apart to fix rust and put it all back together again. The other side if the sword is just take the easy way, patch what's noticeable, drive it, have fun with it for a few years and flog it in. You lose less money that way and its less work to do. You just need to be realistic about your goals of this car, whether you want it to be short or long term. Nothing wrong with whatever route you choose.