Make sure they are COMPLETE and WORKING machines. If they are "parts machines" that you are thinking of getting going, I would be hesitant. In alot of jurisdictions, machines cannot leave the casino "complete". Casinos would take all the "brain boxes" out of the machines and stack them on a pallet. Then ship the machines separately. Problem is once the 2nd hand dealer gets them, is a big puzzle to try and match back up boxes with machines (the box has a Hard Drive in it that contains the GT and GC file for whatever game it came from - some HD's may be blank).. Lost dongles and incorrect machine to box matches happen ALL the time.
Most older reel machines had 2.0 and 2.5 MPU's in them. The 2.5 had video card cooling issues, etc. You'll also need a set of ekeys (or an ekey7) JUST to set the machine up. You'll need a License Dongle that matches your reel strips EXACTLY. And of course the GC and GT thumb drives that go with the Dongle and strips. You'll need the Proper OS that matches with the correct boot chips/proms. Knowing all those minimum requirements, you'll understand a "project machine" is something you'll spend alot of time and effort on and may Never get it going (if it's missing the License Dongle or Installers).
When buying used AVP's, I recommend a Complete, working machine for the AVP beginner. That way you can tinker and learn how they work without getting discouraged that the machine may never work.