Ghost, by what I've heard (which, admittedly isn't much about used Z4s), you're probably right in thinking that the majority of bugs have been worked out with the drivetrain. It's the electrical gremlins in BMWs that concern me. However, if it's well maintained and low-mileage and fairly new, it's probably got a few good years of useful, cost-effective and fun life left in it.
I've always found the few Z3's I drove to be buckets of fun, great sounding and sexy to look at. The Z4 (especially the 3L) just take all that to the next level. I'd love to have one. In that darker-grey colour please with the big sport pkg wheels. Sexy. If he's a 350Z driver, he's obviously not worried about the most visceral, nimble and tossable car available and the Z4 will provide a nice, rare machine with a good sport / touring compromise.