Hi there,
Consider me still a relative newcomer to FreeNAS.
For one thing, you might consider going to LGA1151 for about the same amount of money, since 1150 is somewhat older. 1151 allows more RAM down the road, where you are limited to 64GB with 1150, or what the motherboard specs say. Your choices for hardware seem sound nevertheless. Remember that 8GB RAM is the minimum required, and will probably be more once you go over 6TB of total drives. If you want to do more than storage (VMs and Jails), then that increases RAM required.
Depending on what you choose, you just can't just add a single drive. Not feasible. I'll give my current setup as an example ; 3x 1TB on RaidZ1 = 1,7TB available space.
If I want to upgrade with 3x 2TB, I have to [edited after I read the documentation] put in the three new drives with the 3x 1TB already present, boot the machine and then upgrade. This will replace and retire the 3x 1TB and first silver in the new drives. If I can't for some reason put all three drives together, then I have to retire one drive at a time in the same procedure as a failed drive, which is not the best case at all.
Another upgrade solution is to add another array of drives under your choice of mirror, RaidZ1, RaidZ2, etc. If I understand correctly though, that does not expand the setup seamlessly, but requires to be treated as new network space. Meaning, in my scenario, the extra 6TB is separate from the existing 3TB and can't be mixed as 9TB in contiguous fashion. [Actually, that can be done, but it creates a mixed setup: for the network, it could be a single drive if so desired, but does not change that you now have two RaidZ setups that are unrelated when failure occurs.]
There is a lot of documentation over at FreeNAS if you want to go in more detail, as well as some great How-To's stickied in the FreeNAS forums, including on adding disks if I remember correctly. So don't quote me for a correct procedure here.
FAILURE OF A DRIVE
When a drive fails, you take it out, put a new one in, and it gets silvered in, which, depending on the amount of data and the size of the drive being replaced, can take hours to days, to perhaps weeks. It puts pressure on the remaining drives and hampers performance.
BACKUP
The fact that you are going to have three different saves of all your data is great. Ideally, one save is off site.
If possible, buy different drives from different sources to try and avoid simultaneous or closely timed failures. You could mix WD with Seagate for example. The important part is that the drives are of the same size and similar performance if you put them all in a same pool. In my case, I bought three drives at close to the same time. So maybe I'll preemptively replace a drive a few years down the road even if all is dandy.
There is a lot to learn, so you could do some testing with a first drive and learn a few things, and on a virtual machine too if you prefer, or an old unused rig, which is what I did.