i bought a intel n100 board with used 4gb of ram and an used ssd. then i bought 1 nic with poe for the wan (5g antenna with integrated modem) and second nic for a dmz.

also i tried opnsense but switched to ipfire, because it offers all i need in a much simpler package.

cost was arround 300€. tbh i would just buy a simple mini pc with enough nics, but couldnt find one with poe to a good price.