
A 100mm drop will look very 'Euro', but you're left with virtually no wheel travel thus it'll wreck your chassis in no time. Plus with the Mégane II, the more you drop the rear, the further forward the rear wheel seems to move in the wheelwell. Doesn't necessarily look good asymmetric like that.
I'd say a 60-70mm drop is just still acceptable in terms of wheel travel, provided you get a proper coilover kit (e.g. K&W or H&R) that's properly set up.
As for the dished wheels, I think such wheels on the Mégane in 17 or 18 inch look an inch or two smaller, 'thanks' to the massive wheelarches that tend to dwarf almost any wheel design. But choose your dished wheels wisely and get the tyres as close to the arches as possible (~60mm drop and well chosen wheel offsets), and you've got a real headturner. You'll want at least 8" wide wheels, I think 8.5" is ideal, although even 9" isn't too much if you're going for extra-wide 245 or 255mm tyres.