Look, if a hacker wants to get on your pc and is willing to expend the time and energy to do so, the hacker will get on your pc.
As far as your exposure on the internet when connected your SPI Firewall should be plenty.
Understand that a firewall doesn't protect you from spyware/malware/virus's.
One advantage of ZA or a software firewall is that it can notify you of OUTGOING application requests. This can give you a clue that you may need to look at your pc a little closer.
Personally I have been behind a Netgear firewall for 4 years running no AV or ZA and haven't had a single issue. I do run spyware/malware scanners and know how to practice safe computing. My kids computer on the other had is a different story. I have it loaded and scheduled [since I know they won't take the time].
Knowing the correct answer and giving a correct answer, are two different things