No you would not create a gateway group. You would just create VIPs for your other IP, then sure you can have any specific client on 1 network use IP .2 and other clients on that same network or a different network/vlan use .3 as their source IP when they nat to the public internet.
But that wouldn't be done with gateway groups. If you had multiple interfaces, pfsense will not allow you to put them in the same network, ie this 13.13.13 wan transit network.
As to what appliance - what budget do you have? The sg-3100 can do gig.. But you might want something a bit more beefy? 4860 (end of sale), 7100? I do believe there are some other models in the works as well that might fit you need/budget.
A bit more about your network and plans for pfsense - running packages like proxy/ips ? etc..