Std 18" wheels on a 986 are:
F 7.5 x 18 ET50
R 9 x 18 ET52
Here's an offset calculator so you can see what effect different offsets have on the rim placement. You need to stay as close to the above #s as you can. If not, you may need wild sized spacers (don't want to do that) or the wheel may end up too far inside and will rub on struts and braces. This info should allow you to play to your heart's content.
Wheel Offset Calculator
Regards
Dan