This will help you on tires, I just posted info on going to 18's from 16's and 17's. It's and active topic if you would like to chime in.
Don't know about 18x9's working??
But think of what kind of look you want, if you want that super wide aggressive stance stay with the 8.5 and get the bigger offset I believe. I just ordered mine and forgot what offset I choose; but I got the wide stance one. LOL!