Another is my system powerful enough?

[tecfreak]

pfsense is BSD and not Linux.

OK, 2.5" sata could be a problem for the small NES case. But I would first take some measures before buying some expensive hardware. Maybe there is enough space for the ssd beneath the mainboard.

But how do you want to fit a GTX 750 in there? Even if it is a low profile (KFA) pcb, I can't imagin how you want to do this. (flex)-riser card?
I'm really curious and hope you provide some photos.