We want to connect a Sonicwall TZ-170 to it and give it the public static IP address so that we can setup a VPN tunnel to their main office.
As KewlRobD2 suggested, you should configure the TZ-170 with your public static IP address(es). The WAN bridge will simply forward all packets to the WAN port of your router.
The MOCA LAN bridge will still connect from the LAN side of your router to the ethernet WAN port of the Actiontec. You can configure the LAN bridge either to get a DHCP from your router (per instructions) or you can assign it one of your static IP addresses. This would be done within the TZ-170, so can't help with how to do that.
The Sonicwall's ports are only 100MB. Do you forsee any issues with this?
That's faster that your WAN connection. No problem.
The only issue would be if you are using the switch ports on the sonicwall to connect together Gig-E devices on your LAN. You would be limited to 100Mbps between the gig-E devices. If you do have gig-E devices on your network, put a Gig-E switch behind the sonicwall for interconnecting the gig-E devices at full speed.