That size is going to be a tough fit...18's are a little big for S13s. But that's up to you to decide and try.
I have no idea what the clearance will be for Z brakes or for coilovers...all you can do it buy them and try it. You might need to get spacers later. The Z brake clearance is all about the spoke design (and it also depends on if you get the 26 or 30mm calipers). +40 7.5" will probably clear coilovers though.
use this to calculate offset stuff:
http://www.mercurycapri.com/technical/suspen/woc.html
You are going to need about a 20mm spacer to bring things really really flush. 10-15mm will be a good start, though. Apparently, stock studs are 42mm...you will need the 60mm Nismo studs for the spacers. Or, you could try getting the H&R kit. Check
www.optauto.com for those.
Anyway, start googling
Oh, here's my car with Z brakes (26mm) and 17x7" +40 wheels. The clearance is so tight, I can hold a business card between the "NISSAN" and the back of the spoke.
Eric