If I subscribed for 1 month and stopped would I lose the items/character that are only available with membership?
If your items are equipped to your character you will not lose them, but if you ever remove them from your character you will not be able to re-equip them without purchasing an item unlock from the Station Cash store. The bank space that you have will become limited and anything that you have in the bank above the allowed spaces for a free to play account will not be able to be accessed.
Deputy, A good friend, Enkur just did a test regarding the bank storage. I was incorrect in my reply to you with regard to accessing the bank spaces not open (greyed out) to a player on a Free To Play account. The correction: you will be able to withdraw from those spaces, but you will not be able to store in those greyed spaces. Thank you, Enkur!