Hi Zip. I did what you want to do a couple of years ago -- see pics
here. (I think you will need to be logged in for the pics to show.)
I know nothing about wheel offset, but I can tell you there is no protrusion outside the bodywork of the car. The Trailhawk rims are only 1/2 inch wider than the 18' Limited rims, so I think the crucial factor is more the width of the tyres, rather than the rims. I went for 225/65 (note the spare tyre in your boot is a 225/60 R17). We did try 235/65 in the tyre shop, which looked great, but there was (arguably - without using a plumb bob!) some protrusion beyond the bodywork, so I decided to play conservative. The wheel arch also started looking a bit crowded at that size (almost rubbing at full lock).
After 46,000km in the past two years, I'm very happy with the decision I made. Only issue has been getting rid of the 18' rims -- they're still sitting in my garage!