What parts of car disc brake system wear off?

I am buying the whole car disc braking system from a friend who is using his car disc brakes aggressively (tuned car + speeding + drifting).

So which parts of the disc brake should I definitely replace and which parts don't wear off at all?

I'm trying to see whether its worth buying it from him because it'd be too expensive to buy a similar system brand new. It sells for 400Euro and brand new might be ~1200Euro. I want to replace my factory-installed ones on my car.


Caliper
Brake Pads
Rotors
(any other part I didn't include?)


(looking at this image of disc brakes --> http://static.ddmcdn.com/gif/disc-brake3…

Thank you