I’ve run OPNSense, pfSense, Sophos UTM, Sophos XG, and Untangle on a Sophos SG330 box (i5-4570s, 12GB RAM, 240GB SSD). If you keep an eye open you can sometimes find them for a good price. The SG310 is a bit less powerful (i3, I think) but still has the network module spot.
It’s a Caswell unit...