[FrEaKZoIdE]

Ok... let me try to clear some information here.
Tire size: Ceed runs 225/45R17, of you go for 18" you must go for 225/40R18

Offset: The original 17" are 17x7j ET56 (Diameter, width, offset). The rims on the link are the same width, and have no information on Offset. But 56 is so high, that you will probable have no issues. You can go as low as 38-40 on a 7j rim.
The main issue with 18" is the weight. you will notice a bit of performance drop if they are heavier.

I run mine with a set of Team Dynamics Pro Race 1.2 18x8ET28 (very light and stiff), I have a bit of rubbing, but great looks!
Still on an 18x8, you can have ET35 with no problems, and maintain a nice looking wheel.

To me these are the best wheels you can get, because of their durability and extreme low weight.

 

[FrEaKZoIdE]

Wry 'Cuda,
There is a BIG difference if you run a heavier wheel than a lighter one.
I've had 2 sets of 18", the current one: 8Kg each and a set of Mille Miglia HT3 (12kg).
The standard 17" Ceed rim weighs 11Kg and they behave the same as the lighter 18". The Mille Miglias had worse acceleration and a lot worse braking performance.
The reason is unsprung weight.

I also have a set of 17" OZ Superleggera (~6.8Kg) for trackday use and the acceleration and braking is completely different for the better.

 

[FrEaKZoIdE]

You feel it more on hard braking, specialy on stock brakes.