There's a thread lower down that explains all about the PCV system. I'm not exactly great at explaining it, but it's essentially simplifying the oil system to minimise the potential for leaks etc and stops the stupid EU emissions regulation pipes throwing dirty oil back into your engine. It won't change performance but it makes your engine bay much neater.
The carriers are generally rare on their own, you can buy them along with callipers quite easily off eBay or from a breakers, but I needed it sorted before MOT so I just ordered the carriers alone, new from eBay for about £120. I still had 16s on at the time so was worried about fitting the bigger discs but they just about fit!