Recommend that you read the sticky at the top of this forum's page on spacers.
http://www.m3post.com/forums/showthread.php?t=336209
18mm spacers would give you almost the same stance on the rear as the RAC RG63's (There's plenty of pics of those around to get an idea how that would look). You could also run 12 or 15mm just fine also (on the rear). It all depends on how much of the boy racer look you want. For reference the rear ZCP wheels are 10" ET25 and the RG63's are 10" ET22
The fronts are a different story. 18mm spacers would be pretty aggressive. Apex is selling their 9.5 ET35 ARC-8's as a no spacer fit. A 7 mm spacer would put you in about the same stance position as the RAC RG63's, but could be an issue because of the hub pilot being lost. 10's or 12's could work but you'd have to be careful with tire sizes (wheel turn clearance). I'd be inclined to try no spacers on the front or maybe a 5mm spacer (as long as the hub pilot still had enough contact). For reference the front ZCP wheels are 9" ET31 and the RG63's are 9" ET22
There are a number of alternatives that don't require spacers and allow you to maintain width stagger. The Apex ARC-8's appear to be a really good value in an 18" wheel, the BMW ZCP wheels are very high quality at a moderate price, and the new Morr wheel looks like it will be a very good wheel (for the cost).
Since you live so close, I recommend that you go to the Dinan open house on 9/25 (RSVP on the Dinan website). There will surely be lots of cars there with many different combinations of wheels. I'm sure people will be happy to share their setup info with you.
