Sonicwall TZ series install dual WAN

Hi,

I am after some pre sales advice that would be appreciated.

We are going to install a Sonicwall TZ215 on dual WAN topology.  At present one ADSL has a static ip and the other dymanic.  I know the TZ215 supports DDNS but is it best practice to have both ADSL line on a static ip address and is a single ip address enough ?  ie does each router / firewall node need a public ip address on the WAN side ? 

Thanks in advance.