Small PC server instead of hub/port replicator for ultrabook (MacBook Air)? Which OS?

[Insider]

Here's the project: I was thinking of instead of getting an external hub/port replicator for my MacBook Air (a good one is $100 minimum), why not get a separate, fully fledged small PC server (I'm OK with a price of up to $200, you can buy a Kangaroo PC, Intel NUC and various other boxes in the price range. On a side note, if you want a 256 GB SSD in a MacBook Air/Pro instead of a 128 GB one, that would be $200 extra, too, that $200 is better spent at peripherals, hub/port replicator -> small PC server, especially), the PC server would connect to the MacBook air via fast (ac) Wi-Fi. The PC server would connect and share Ethernet Internet and various drives, such as a HDD/SSD for files, USB flash drives, SD cards... with the MacBook Air.

First, is this a good idea at all? I'm not a UNIX expert, I understand there is a learning curve for all the three, major BSDs I might be interested in taking possibly for other, future projects.

There are also things like 'ready made' (FreeBSD based) FreeNAS (for files) and pfSense (for network) and I seem like to want a combination of these two in one box, which might or might not be a good idea.

Your thought? How would you do it? What level of expertise is needed? What's my best bet if, for the time being, as a beginner/not so technical user, I want something that 'just works'? The recommended OS can be BSD or Linux based, or else.