On board should be fine but if that dosen't work and you don't have a free card lying around get a simple intel gigabit adapter. because of the nature of them you less likely to have driver issues.
With regards to Server 2012 R2 or Win 8.1 with Storage Spaces there are some gotchas. Storage Spaces on Win 8.1 is feature limited compared to Server 2012 R2 - you can't set storage tiers for example to use SSD's for tier 1 and SATA for tier 2 which you can do on Server 2012 R2.
To get nearer to ZFS like functionality you would need to use ReFS instead of NTFS. It's not something I have recommended anyone to use for mission critical data just yet, I'm waiting to hear if anything will have changed (matured) with it under windows 10 or Server vNext.
For a home set up I would probably go with one of the open source options unless you have MSDN licences or similar you can use for Server 2012 R2.
Yeah, my first FreeNAS build was on a micro-ATX Gigabyte board with a left over AMD Phenom II X4 960t cpu with 16gbs of AMD non-ecc memory. It worked perfectly fine. I upgrade later to Intel cpu because of the lower power draw and because I can't help but tinker with things. I sold the AMD parts to a friend for a low budget gaming build.