YOu don't need to purchase a key - Hive signer is just a way of signing onto the platform. It's bloody confusing I know.

RC credit you need for transactions on the blockchain. It refreshes, but the more HIVE you have, the more you can do things like vote and upload photos - sometimes we don't have enough to do the most basic things which is frustrating when you start out. Feel free to ask any questions though and I can help out.