If it is cost I would go 225/40R18. The bit of the arch you might have to fold is at the bottom of the rear next to the pinin badge and you may need to cut back the end edge of your LE skirt there as well. It's because when you compress the rears the wheel goes upwards and forwards.
If you have a rear zender skirt (which I doubt you will have) you will need to cut and shut that so it is flush with the arch line
On the fronts if your arch liners are in the right place you may have to cut back the grilles in them or let the tyre do it over time
If they're just hanging out then the tyre might rip them out if you're not careful.
With 225/40R18 your speedo will read about 3% lower than reality I would guess.
Offset makes a big difference on how large a tyre you can get away with.
On the front the best thing to do is put it on full lock with your current tyre and measure bearing in mind the wear on your tyre. Look at the difference in radius of the two and then you can work out whether they will fit or how much they won't fit by.
Apparently the clearance gets tighter with the car jacked up slightly btw (not that you are likely to be going over a high speed bump at full lock)