Sounds like a Z4 is perfect for you. It rates out much higher on all of your concerns than the Z3. I now drive a Z4 3.0i and had a Z3 2.3 a few years ago. In between I had a 325Ci. So I can compare with experience.
The Z4 has answered every question I ever had about the Z3. Plus, moving to a 3.0 gave me the power I had been looking for. The trunk? MUCH more usable room in the Z4. This car is rock solid and handles like a 2003 car should (worlds ahead of the Z3).
It is a much more comfortable car than the Z3 with the larger interior and better suspension, but without any compromise to performance. They are dealing on these cars right now and the lease rates are very low. Might be your answer!
Rock Chalk -- JAYHOX!