I'm about to pull mine apart soon.
I bought a belt & both pulleys to replace at the same time.
I didn't get new hoses as these didn't leak.
Regarding the O-ring, I would take the one you have to ACE hardware and just get new ones the same size, probably made of Viton or Kalrez.
Great suggestion regarding taking the o ring to Ace.
Regarding the hoses, mine were kind of trashed, so I threw in hoses too. I just made sure to buy a pump with out a pulley so I could put to use the pulley and remover/installer tool I bought for the original job.