You're considering several different categories of car. The 240Z is the basic, no frills, carb'ed "pure" sports car that Nissan intended. A 280Z is essentially a refined 240Z, with EFI and its associated blandness. A 300ZX is more like a modern car, smooth, quiet, even more bland. The Porsches are a different kind of car, more "engineered" than the Z's, which tend to take the simple route to accomplish an objective.
These are all "old" cars though, compared to the computer-controlled vehicles today. You're going to have to learn on the fly to keep any of them running well, unless you find a shop nearby that can work on one. You might consider that in your choice. Find a shop that can help, then choose a car that fits the shop. Or pick the car you want and learn how to make it work. Some of the ideas you're throwing out are a bit exotic and knowledge-intensive, like the RB26 or the twin turbo 300ZX. Even a carb'ed 240Z takes some learning.