They will fit, but most likely you'll experience some rubbing either when you turn the wheels or when you hit bumps and potholes.

Aesthetically, they only look decent when complemented with a body kit and the sorts. 18's on its own looks pretty ridiculous.

Yes, it would slow down the car, if not by the weight of the wheels, then just by the sheer fact that it's harder to turn a bigger wheel than it is compared to a smaller diameter wheel.

If for some strange reason you still feel the need to get a set of 18's, make sure that you get a 18x7.5" wheel with at least a +42mm offset. Mate that with 215/35/18 tires.