The answers to your questions would take a LOT more space and time than we have here right now.
You're going to need to do some reading on the 70-78 Z cars so that you understand the differences in the years. If you want a 280Z, that's a 75-78. The engines for them are all basically the same (L28 fuel injected), so there's no real reason to choose one over the other. 75 was the lightest and least-loaded down with safety / emissions stuff.
The two rules when buying a Z car: Buy the best one you can afford, and if there's rust, walk away.
Once you've narrowed down what you're looking for, pop in and we'll help you whittle down your options.