I looked long and hard at picking up a '98 or '99 Z3 2.8. For the money they were among the best values in my opinion. I had personal experience with the 2.8L drivetrain and could attest to its solid and sound build quality and its smooth, satisfying and linear power/torque and was keen to have it in a car smaller and lighter than my 328i. My reading turned up more quality control problems with the original Z3s ('96-99 I believe before the mild facelift) than with the 3-series sedans -- of particular note, interior trim bits of low quality. Still, with fewer electric bells n' whistles than my sedan, I figured it was a pretty safe bet.
I came very close to a "James Bond blue" '98 and still -- on nice days especially -- wish I had gone for it. The I6 makes a nice snarl and exhaust note with the top down and overall the car just felt really right.
Plus, I like the wider-fender look of the bigger-engined models better than the 1.9's and 2.5's (2.3) Z3s.
Bottom line, for the sake of a couple grand (at the most), the 6 cyl cars are so good, I'd look for one of them.
(sigh... ahh, to have a roadster...)