Hi,
I have used a second hand ameda pump for my baby. I did not buy new spare parts, because the parts look quite well maintenance when I brought it. However, I used my own bottle, and sterilize all the accessories after each pump.
I think you have to assess the conditions of the spare parts before use. If you are really scare, just change all the accessories that may have contact to your breastmilk. It will still be cheaper to pay for the motor, than to buy a brand new set.
However, you should consider to buy one that still have at least 2-3 months warantee from the time you start to use. This is just to play safe in case the motor not working and you still can have free servicing. My Amenda pump really out of order at the last month of warantee, luckily I still get FOC servicing before the warantee over.
Hope my experience helps. : )