If this is your first bike, you might want to avoid anything too extreme. Something like a Ninja 500 or GS500 is pretty close to a Z28, performance-wise, without being something that's too much bike for a beginner.
I don't know Kawasaki nomenclature too well....So I couldn't tell you which ZXRZZZ 1100, 1200 etc... that I'm talking about but:
Git yerself one o' them big green Eddie Lawson lookin' things ~ you know ~ Kerker exhaust, big ol' pipe braced swing arm, lotsa tire and lotsa motor, wiggles & shakes when you ride it hard , motor sounds kinda raw & phat! yeah that's the ticket...!
This bike exemplifies the spirit and style of a Z28!
Definitely a Bandit 1200 (which I sold a few years ago and now miss). When people who didn't know bikes but knew cars used to ask me what type of bike I had, I always said it was sort of like a Z28. Big-block power, cheap as hell (for an engine of that size and power), very old-tech frame and suspension, yet reasonable capable once tuned and/or upgraded, and simple as dirt to maintain without a very complete set of advanced tools.