Kawasakis are generally very reliable. My 97 ZX-7R has upwards of 30,000 miles on it, including track miles, aggressive street miles, some stunting, and a handful of crashes.
the early ones were also very reliable. The biggest problem you will run into is parts. My friend has a couple of 91, and 92 7's, and I know he often has trouble replacing parts, particularly bodywork.
Will this be your first bike?