Appropriate Wheel Offset for 18" Wheels?

Thinking about possibly just getting some quality wheels and using a spacer... I'm kind of liking the Enkei PF01. I kind of like black but silver is cheaper and will get here sooner if I order them. I know I'll need spacers in the rear, I'm just not sure how wide.

http://www.summitracing.com/parts/enk-4608956635bk

I think they should look good with big brakes.