Chrome is just aluminium plated. I've got 18" wheels with very low profile tyres, ive written off one and had to take a sledge hammer to 3 to correct pothole damage. I'd suggest you stay with 17" if your roads are bad. Or if not, make sure you order 5 so uve got a spare.
If you do go for 18" and u want them to be strong yet light your gonna be forking out a lot.