There are good wardrobe door sliding systems you could look at Boolders if you fancy putting it right. They're not at all difficult to fit.
Otherwise, yes, 18mm is not enough to mount conventional hinges. You'll need surface hinges of some kind. I've used ordinary kitchen cupboard hinges before. I would guess you would need four on each door though.
They need letting in to the door, but if you want something similar, think about these.......
http://www.toolstation.com/shop/p13965