I think your coverage is fine. I would recommend one more tool however to your arsenal, malware bytes anti malware. It too, is a free program. With the tools you have now, you are perfectly fine. There is no need to purchase additional protection. However, if you do decide to get the zonealarm antivirus, make sure you uninstall avg first.

Remember: You can only have 1 firewall turned on, 1 anti-virus, but you can have many antispyware programs. As long as they are not all having there own tray next to the clock. Basically, you are fine and ready to go. Just thought i would offer my advice to you. Good day sir, and keep learning!!