Since you're just trying to setup a home networking, I don't think you need to invest a NAS for storage.
1. Instead, you could try setting one of your pc as a server. Backup all your files inside that pc's HDD.
As for your external HDD, you can't treat it as a standalone HDD, it needs to be connected to a PC in order to function.
You can connect that external HDD to your server PC, any backups around the network will thus have to first pass through your server to access your external HDD.
2.Connect your USB printer into your server pc. That, anyone over the wifi network is able do printings through that computer.
Networking at home, Networking for file storage and printers