How many XYM do I need to run a node?
You do not need to own XYM to run a node. If you want to profit from running a node, run a peer/dual node, allow other accounts to delegate harvesting on your node, and appoint the beneficiary account. Your cost will be hosting the node. Refer here for more details.
I can't reach my peer node with localhost:3000.
Peer nodes don't have API. Only API or DUAL node would allow you to do that.
What does "The wallet could not load data about the network currency (e.g. symbol:xym), please connect to a valid node" mean?
At the bottom lefthand side of the wallet, there is a dot with Node next to it. If it is red in color, click on it and select a different node from the list until it turns green. Otherwise, try log out and log back in. If the problem persists, check the firewall and proxy settings.