So, I hope you'll post those picts before you buy it. $5K is a lot for a Z unless it's in really good shape. but hey maybe you're onto a really good deal. just remember though if there's some rust, likely there's a lot more you can't see. $5K I'd expect a lot, like perfect dash, seats and vinyl. low miles and well running engine. Rust really puts me off, so you'd need to decide how much to expect, and what you can tolerate. It would be a very good idea to put it up on a rack and have the underside inspected for rust, leaks and any other problems.
I wouldn't worry much about the timing belt, the Belt is a Chain, and though it's supposed to be changed every 60K miles, I've seen them last well over 120K miles. though over time they do stretch and can cause less efficient running.
I would suggest you read through all the threads you can find about rust and remediation before you buy the Z. since likely there's a lot of hidden rust, you don't know about.
Phar