I would recommend FreeNAS as this is what I've used, it has great community and serious company behind it and it seems to be more up to date with FreeBSD releas

I would recommend FreeNAS as this is what I've used, it has great community and serious company behind it and it seems to be more up to date with FreeBSD releases which is always a nice thing. When it comes to build recommendations, the main thing seems to be about RAM - you dont need a lot of it, contrary to popular belief, but you might want a lot, as ZFS loves large amounts of RAM (it can run with a smaller amout too, things like prefetch, and others, can be disabled). More important, you want the ECC RAM, given the ZFS unability to control memory errors. Beside that, you're free to do as you like, with a small advice of building at least RaidZ2 or even RaidZ3 (there are a lot of horror stories about people running RaidZ and loosing more than one disk in short time period).