I vote keep it. Like you said, you've got most of the kinks worked out. If you buy a new car and start fresh you will have to start all over again. What exactly do you want to redo besides the paint? If you bought a new project, you would likely have to redo the paint on that as well. Not to mention, problems ALWAYS seem to be hidden in new projects, whereas you know your car in and out.
I feel you on that dream of buying a nice, clean, rust free, bone stock Z out of California though. It's something I will probably do as soon as I get the chance.