The IPMI management app usually lets you mount an ISO over the network. Just download the ISO for FreeNAS, mount it using the IPMI software and then boot to that.
Looks like a solid build, You should be all set, just make sure the ECC ram is UDIMM (not RDIMM), or your motherboard might not like it.
For boot, anything is fine really, if you use an SSD or M.2 drive it can act as a cache and improve performance. Here's a really good FreeNAS guide...
In an absolute sense, there is no real reason you have to run Plex on a separate machine from the NAS. Since your NAS is server grade hardware (more reliable, always running), it makes a lot of sense to run Plex there also, since it will save you power, and you have plenty of free CPU resources...
Your HTPC specs look fine - though I personally prefer the Fractal Design HTPC case, as an optical drive on a HTPC is sort of redundant when you're planning to do all your playback off the network. The Fractal Design Node 202 is slightly nicer looking and comes with PSU, though you can buy...
If you want your server to perform well as a NAS, anything will suffice CPU wise - but for Plex transcoding you should have a few more threads. I would go Xeon as I recommended above and below, as it's a server grade chip and is meant for a higher duty cycle. With what you'll be doing you can...
4K/HDR can be played back on a PC with the latest gen Intel processor, or with any machine using a dedicated graphics card (GTX 1060 or higher) if not copy protected. It's really just about having the capacity to handle h.265 HEVC and an HDMI 2.0+ device that can send HDR metadata. You can build...
Feel free to do so, we are no longer against HTPC/ripping discussions here. That's largely my doing, as I could care less if it irritates the studios. We are paying for our media - and should be free to use it however we wish.
The major benefit to building a more powerful xeon based NAS/Plex server is that you can consolidate everything in one box to lower costs.
A NAS is just a file share - there's nothing special about it. One day, when Android based boxes like the one I linked become available with the chip...
Plex's native playback method utilizes transcoding, while also offering vastly more limited video quality options than Kodi. It's great for what it is meant to be (accessing your media from 3rd party devices and when not at home). Inside your home, Kodi is clearly superior.
Plex is the easy...
My 5 cents. Don't waste time/money on plex or any transcoding solution. You're giving up picture quality, which is the whole point of ripping blu-rays and playing them back natively.
Invest in the right endpoints (Kodi players)like this: http://****/2eX5e0w and you can play anything back...
This is possible, and doesn't require transcoding. You basically have to use software to rip (AnyDVD HD is the most popular) and then store on a NAS or PC's SMB share. From there, you just need a device that can play back blu-ray's codecs natively. The cheapest solution for this is a Kodi box...