I have a 93 Mazda Miata (NA chassis code). Had been doing research on them for over a year, and realized that they are slowly going up in value. They're fun, cheap cars, and IMO will eventually become classic collectibles. Other motor heads have agreed, but just my 2cents.
If I were you, I'd stay away from the 300zx, as that was one of my candidates, since i've heard they are a BI*** to work on. there's no room under the hood!
Also, the cars you listed, the z car is probably the best candidate, because it is a NICHE car. meaning that, like mustangs, corvettes, miatas, and i'd like to include Aston Martins etc., they have a following of particular people. The integra and 240's... they are like civics. everyone and their moms have them now, thanks to fast and furious movies. not particularly good cars to flip. you'll never get your money back.