Motorola NVG510 have a built-in firewall?

All router in a way have a "firewall" that will block most common attacks just because they are too dumb to forward the packets to the right nat address.

This one have a very basic firewall that will protect the router itself against some of the common attacks.

Unless you are going to place the machine in a DMZ or portforward a lot of ports you should be able to get by with just virus software. Client based firewalls tend to get in your way more than they protect you in designs like yours.