I'd shoot for a Duo-R (or Duo-RX if the price is right), as they aren't much more expensive than a Duo. Some believe that they are a little more reliable, but it doesn't really matter since HOP-M3s are cheap and readily available. All Duos have a v3.0 system card built in, so they can play 98% of the PC Engine library right out of the box. You'll only need an Arcade Card (Duo or Pro) to play the handful of Arcade Card games, and Games Express releases should come with their own system card.