Is there is a game/anime/manga which is in English in one form or another (either Dubbed or Subbed for anime or officially translated/localized or fan translated for everything else) which would be a good entry point into the series?
How about this youtube channele? You can watch promotional videos and radio programs that you can watch for free; original, Million, Cinderella, Shiny and SideM are all treated. Anime and specials are distributed depending on the time.
Of course, these are legal content operated by copyright holders.
I think the 2011 TV anime series is the obvious choice. It corresponds to 2nd Vision games and the classic breakout period while the numbers of idols were still manageable (it has 11, if you count twins as one). The anime that came before was awful. The anime that came after is often a movie, spin-off, or just downright fruity. The 2015 Cinderella Girls series is okay, but not any better than the original.
I read most of iM@S manga and I cannot name one that's definitive.