RAM should be fine. 32GB is plenty to run reliably. I have several systems with 64GB of RAM that have 365TB useable. Not sure about those HDDs. I haven't done much research on SMR drives in a FreeNAS system. Perhaps somebody else can tell you more about that. As far as a boot drive goes, grab yourself 2 16GB Cruzer Fit USB drives and use those for a mirrored boot drive. Real easy to do. That mobo only has 2 USB ports total, so I'd snag something like this: http://www.amazon.com/Motherboard-Female-Header-Adapter-Cable/dp/B015F6QXKO/ref=sr_1_2?s=electronics&ie=UTF8&qid=1457296058&sr=1-2&keywords=usb+header+dual Attach it to the header inside, and leave your boot drives in the box, which frees up your limited USBs on the back.

I built a box very similar to yours but with the ASrock C2750, and 6TB WD Reds. One thing to look out for is that your mobo has 6 Intel SATA ports and 6 Marvell SATA ports. Do not use the Marvell ports for your set up, or your customer is going to have nothing but trouble. Marvell controllers are notoriously bad in FreeNAS set ups, and although you may initially see all of your disks, you best believe those suckers randomly stop being able to see them. I read that folks have had success only using them in Mirror set ups, where 1 disk of the mirror is on the Intel side and the other disk is on the Marvell side.

I'd just search usb2 header and look through what kinds of adapters you find. Here is one that seems to be what you described.

https://smile.amazon.com/10Pin-Motherboard-Female-Header-Adapter/dp/B015F6QXKO/