I need some advice..

Hello, and thanks for taking the time to read/reply to my questions. I am building my first home network, and I want to do it right. I am using a cable modem, router, and networked printer to connect a two desktops and a laptop. I have all of the security features enabled for the router, but would it be a good idea to still run software firewalls on each system? I would also like to purchase a external hard drive to share data and backup all of the PC’s to one location. I have considered a network connection hard drive, but if in the event that I would need to transport the drive somewhere, or just connect it to one PC, would it work properly, or should I just go with a USB 2.0 connection?
Any suggestions?