This was recommended by a member named Orion (Brian) for an S13:
For a decent look, and easy fitment:
7.5", +10
8", +15
8.5", +20
9", +25
Its just a guide though. In your case, I'd do 8.5" +20-25 in the front and 9.5 +30ish in the rear. Now that I type this, a 10mm spacer in front and a 5-10 spacer in the rear should look pretty good. But I'd rather have the offset and lip built into the wheel.
BTW, I think the 18" Sportmax 962 you see a lot come in 9.5" +38mm so if you see a pic of those, you can compare.
And heres a good calculator. You can use 8" +35 as the closest clearance to the strut the S13 can take and just plug in numbers of the wheels youre considering. I know the rear has more room, but I dont know how much. I'd use 9" +20 as starting point for the rear.
http://www.1010tires.com/WheelOffsetCalculator.asp
Heres 17x9 +20 F/R for comparison.
http://www.splparts.com/Parts/...G.htm