A 39 offset a the rear will be fine as the rear brakes are tiny and nearly any design will suit. Bear in mind the S15 brakes (spec R) stick out a bit so some wheel spoke designs won't fit despite having the correct offset. I tried a set of deep dish 18's with a 35 offset and they weren't even close to clearing the brakes. If you do run 39J on the rear, as I mentioned before, you'll still need to roll the guards and get rear camber adjustable arms if you intent to lower your ride and run 235's or bigger. Without the adj rear arms I could run 245's without rolling the guards, but negative 3 degrees camber with 245's doesn't have a much bigger tyre contact patch than the standard 205's with correct camber.