19s fit on a clio with very minor arch mods.
For 18s, I'd remove the arch linings, Roll the rear arches, get a 215/30 tyre and lower 40mm.
18s fit on a clio 1.2 and lowered 60mm with no arch mods, DJJOHNNYB has 18" Brooklyns on his. IIRC he has lost a bit of full lock, and dont think he can have many people in the back.
Id go for 18's, lowered 40mm on coilovers, not springs though. This way it wont look silly, wont be too compromising on the ride quality.. Coilovers are also thinner which gives you more room
The problems come when lowering TBH, 19s fit on a stadnard car offset dependant, as long as you have coilovers, its just you cant lower it or have people in the back.