I’m not sure where you’re looking exactly but I just went to their site, products > Nas & IP san > personal &home user. That’s where I found those. The 420j is likely arm based so it would be nice and low power.
All of them are going to come with DSM as an OS/web interface. The difference is how much throughput you get and how much CPU you get. The highest end ones can transcode video for Plex. You seem to only want file transfer capabilities so no need for a higher CPU. Even the lowest end one will saturate a gigabit connection just fine.