According to the Amazon page, the Japanese is stereo. Is this right? Is the English version have 5.1 available, like One Piece does?
No, the English version doesn't have a 5.1 Surround track. This is probably because the original Japanese was released as only Dolby Stereo and the wasn't a way to create a 5.1 mix from the source (I have no source to back up that claim), and the show is pretty long so it's unlikely there will be a rework of the audio unless there's significant interest.
I've also looked at the HK release of Monster and that is also Dolby Stereo.