This is good general guide that helped me when I first started researching offsets for the z
It outlines the range of offsets and width that fit at stock height and camber. It is just a general guide as height and camber will also influence fitment. Generally speaking you can go more aggressive if you are lowered and or have more negative camber.
Option Zero made a great suggestion. If you don't have that much time to invest in researching what fits, just go with his suggestion. Whatever wheel you buy make sure that you find out if the wheels will clear the brake calipers.