Yeah, it is a good idea to have them done right. Otherwise, you may be in the same situation sooner than later. Unfortunately, I think I am going to be in that sooner boat.....
You are probably better off if you do use one but when I did mine I just cleaned up the case and cover VERY well and laid a bead of silicone in the groove. No problems to report thus far and it has been a few years.
I found the 18 x 9. They are a little steep IMO. I think I can find them cheaper.....
http://customwheel.com/custom_wheels/product_info.php/cPath/21_235/products_id/11925
^^I was at a swap meet in LI last summer and almost bought these exact wheels. I won't blow up this guys spot and say how much they were, but I regret not buying them. If the tire size wasn't so far off, I probably would have.
Yeah, I was running 285/35 rear and 245/40 rear. 35% of 285mm and 40% of 245mm is VERY close. If I used the same aspect ratio between both, you would visually notice the difference. It would look like dookie.