Thinking of buying Xbox One - Need refresher on features.

[I LOVE GIGGS]

You only need one Live subscription per console. You can use more than one account (I think the maximum is 10).
Buy 1 TB edition. Games are ~45 GB and OS takes a considerable amount. You would want to keep games that you play from time to time like sports games or online games like COD and Halo.
Halo MCC for example takes 70 GB from the hard drive abd I'm keeping it along with NBA 2k15 which takes 45 GB.