The SSG140 has router features but will always require a security policy of some sort to permit traffic. There is no "packet mode" on the SSG series as in Junos. Also the router features are more limited.
So first you would need to confirm the scaling of the route features is good for your intended network. Also note that most of the interfaces are only 10/100 on this device.
For the deploy as a router the simipleest approach would be to place all the interfaces into the default "trust" zone. This zone allows communications by default between trust interfaces. So there would be no blocking of traffic out of the box.
Certainly this can do basic layer 3 interfaces and static routing if that is all that is involved.