Somewhat of a misnomer. Yes, ZFS likes RAM, and yes it’ll use a lot if you let it. So the moar the better, obviously. But to suggest anything under 8GB is unworkable is a bit in the dramatic side. You may not get optimal performance in all cases but the system will function well enough to saturate a Gbps LAN and honestly if your budget is $600 with some found rust I doubt OP’s requirements are to stream several full bitrate 4K videos simultaneously.
Are you talking about postfix filters?