If it's a convertable and a z that you want. and you want an S30... that looks like a respectable buy for a 6000 dollar Z as far as finding one in good shape with good paint and an easy to drive carb setup.
The fact that it's convertable, whether appreciable or not, sorta lends itself to the price as a good selling point, because it's custom, looks decent if not almost dealer-option like in this case, and it works/doesnt leak (or sew it seams! LOL *ahem*).
look at it for rust in the common places, and if it's solid, I'd do it if it was one of my better options. I can appreciate the work and the look of it, but I'll take a hard-top any day.