First of all, I just want to say that I dont have a roadster
but I would assume it's suspension is not too different from
a coupe. Now, the most used coils on our Zs are KW V3,
BC BR, TEIN Monoflex and Stance GR+ (I apologize if I
missed a commonly used coil). Now, if I had a roadster, I
would get whichever coil just to achieve a certain drop you
are looking for, get a camber kit, and if you wanna use stock
rays, I would get spacers. Last time I was on stock rays, I
had a 15/20 setup but I'm seeing tuners using 20/20, 20/25,
and even 25/25. It's up to you how flush and low you wanna
get. Yet again, I do not have a roadster, just feeding our
forum members what I know and have gone through.
--edit--
Sorry if my post looks funky, I posted using my phone.