The Kawasaki air-cooled inline 4's were among the most bulletproof motors of their day, IMHO.
The GPz was the boy-racer of its day.
I owned an 82 GPz 550 years ago - that was the first Kawasaki single-shock rear for the street. Great bike.
I think the 82 GPz750 was still twin shocks?
In any case, if the candidate is in good shape, I'd go for it in a minute, with no hesitations.
Good luck, and post some pictures if you get it.
Kirk