The last thing you want is a pro player or Midi File, as you will be hearing the player not the instrument.

Think how keyboards are demoed, they always choose songs and instruments that match the original, and even if the sounds are not that good the punters ears fill in the missing parts as it references the original.

The best is to play something out of the ordinary, and if the sound is any good then it should still sound like the real instrument, (Just badly played) the same as playing a real instrument badly will still sound real.

Bill