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you can go with 8gb of ram (you can always add later when you can afford or you actually need it ) if you dont plan on running full os vms windows ... i had /will have a similar setup on proxmox running pfsesn and portainer for dockers (running unifi controler and pihole for testing) ... plenty of power and ram even with pfsense passthroug with 4gb ram allocated to it (pfsense can run fine with 2 gb)
if you go barebone then 8gb ram ...is enough ....also you can get a normal sata SSD .... way cheaper/ cooler and you will not notice the difference
i would suggest going for the unit that has the side fins or the hole... more heat dissipation power.
all will have the gap between cpu and cooper block ....bigger or smaller... so for best thermals you will need to go grind or shim way.
i would also suggest go i226 .... why not buy the latest gen of nics if the price is the same ? sure is not supported yet on stabel pfsense release ...
same for CPU ... why go j4125 when you can get N5105 or the new J ? ... difference in price is not that big but performance is.