indeed, go 225/40/R18.
i believe 45 wont fit because of the back wheels. with 215/40 or 225/40 the wheels are pretty close to the wheel arch.
also: (about 215/45/R18)
- not legal in most countries. 225/40 will be generally accepted (because only 1,29% bigger dan 215/40, with 215/45 its already 3,3%)
- your speedometer wont be correct (you will go faster in reality then on your meter)
- decrease of performance because of bigger tires. (acceleration will be slower)
+ I'm afraid they will rub in the front too when turning. The rear side of the wheel well is a very tight fit with standard setup, I have 215/45R17, and there's propably max 1cm free space when wheels are turned to full lock. 215/45R18 would have a 1,27cm larger radius than my setup.