Cost: depends on what your're looking for, a project or a runner
Performance: again depends on what your looking for, track car or daily
Fuel efficiency: please its a 40 sports car. It is very possible to have great mileage but that is probably very few peoples first though
Reliability: depends on what your looking at, solid body, strong engine etc. Most people who buy a car on here have plans to mod the thing anyway so its not an issue since they are going to make it reliable themselves.
I already got my hands on my projects, so if I were to buy another Z I would want it to be a runner, not have issues, working etc. Although it would be hard not to do minor mods like shocks and bushings etc.