Yeah the CPU did worry me too. I think the issue is I don't know what I need. Will I need transcoding or will I want to run Unraid? I just don't know.The F4-212 is not just 2GB, it's an arm based quad core @ 1.7ghz
How about the F2-424 (Intel N95 quad core @ 3.4Ghz / 8GB RAM) its £60 more than the F4-212 but it seems a big step up in CPU/RAM and only sacrificing 2 drives overall, assuming you stick with the D4-320.
The F2-424 will let you hardware transcoding (Plex) and would allow other OS' such as Unraid in the future (that is paid, but very much well worth it).
Part of me thinks to just get the cheaper one (F4-212) and see how it goes, but the other part of me keeps thinking of the old saying "buy cheap, buy twice".