18" wheels will fit, it's the width and offset you need to be mindful of.
The Handling Package wheels should be 18 x 8 +55.
For comparison I run 8" +46 and have no issues. If the new wheels are around that you should be fine.
2012 YB Focus SE Sedan [featured in Performance Ford, Sept '14]
1992 BW Grand Prix SE Sedan [restomod project]