Hi. The game is pretty big, so its really hard to give a full list of everything it includes. The story and the choices are the main thing. The game has 9 main characters and dozens of side chars, so there's a lot to do.
Yes, the $ 8.91 gives you access to the full game. It's not finished yet, but all the future updates will be free for you after purchase, so you can update whenever you want and play the new content as soon it gets released or when you prefer.
You don't need Steam. You can download the zip file, unpack and run the .exe file and start playing. Although, you'll receive a Steam key if you want to redeem and play from Steam when the game releases there.