Each to their own I suppose, but I wouldn't contemplate fitting s/h braking components or tyres etc. Save a bit of cash, maybe but how much do you value your life...... 
After the first journey on your bike, all components are second hand!
Maybe, but at least I know the history of them.
What history would you be worried about with regards brake disks? if they are damaged it will be spotted.
I think I'd like to know whether the discs had been removed from a crashed bike. And of course there's room for bulls**t in that question. There could also be hairline cracks that cannot be seen by the human eye. It's pure common sense (my opinion only) to replace old brake components with new. It's just not worth the risk. 
If you were to buy a second hand bike, would you replace all the disks?
Your existing disks could have a hairline crack! How would you know? Most cracked disks I've seen have been as a result of "proper use", not accident damage!
Risk management is always a good idea but realistic risk assessment is a very important part of risk management.
Still, if you're happy not buying S/H disks then that's fine for you.
