I had the 2nd generation Outback H6.
I used 17 x 7 with a 45mm offset and 235/50 Bridgestone RE040's. Ride was a little harder but not much, but the handling and the looks were so much better. I think 18's would be going a bit far if you want to use it off road, but possibly fine for on road only use.
With the 235/50 tyres on a 45mm offset there were no rubbing issues except for a small piece of plastic that was sticking out from the under motor plastic guard. Just cut off the small piece (about 10mm x 15mm) and everything was perfect. Had about 10mm clearance all round with both locks of the steering. You would not be able to use this width tyre with anything less than the 45mm offset as it would probably rub on the inside.