you will have scraping problems if you don't get the right offset and if you don't have the right struts. . .there's no wheel gap regardless when you lower the car, adding 18in's is just for looks because 17's more than suffice. . .when I lowered my car with eibach's on 17's, I had .1 inches after the springs settled between the actual tire and the fender. Wasn't very fun to use as a daily driver, so I raised her back up. . .
Eibach's lower a car 1.25, HR about 1.5. . .use the SEARCH option to answer some of your other questions. . .