The TZ100 is an excellent firewall. I just purchased two of them to link my office to my servers in a datacenter.
I recommend getting the TotalSecure bundle. It's more expensive than the base model, but you get content filtering, gateway antivirus, antispyware, antispam, and Intrusion Prevention.
In my experience, if you turn on the antivirus and antispam, you won't be chasing any viruses on that network. That could be a good thing or a bad thing, depending on how much business you are getting from chasing viruses!
It also includes 2 remote access VPN licenses and 10 site-to-site VPN tunnel licenses.
If you have any specific questions, post here and we'll try to answer.
Eric