Thanks for the thorough response! I've read enough to figure out that I'm going the docker route (one of the big reasons to switch for me as I use CP, Sonnar, PlexPy, PlexRequest, etc.). My thoughts were initially to run something that would play them directly from the HDMI out of the motherboard (assuming there is one) but it looks as if passing video through onboard may be an issue so I'll look into it.
Couple things in response to your other points
Given that I try to have most of my users stream original when possible, I typically don't transcode more than 2-3 streams at a time. Given this, I was leaning towards a 6000-7000 passmark cpu (a la i5-6400~ or so) but now that you say BR rips take more to transcode no the fly, it's concerning. Maybe I'll go with something stronger but I'm attempting to keep this budget relatively tight
PSU 550-650 seems to be a lock. If I were to get an extra GPU, it would strictly be for HTPC media play back (something like a passive Nvidia 720)
Ram is cheap, 8 or 16 will be the target
Do newer MBS and processors typically support IOMMU or VT-D?
On the topic of cache, from my understanding, those who use it typically purchase them and use Mover to send files to the array once per day, lessening the usage of the HDDs. Given that this will primarily be used for PLEX, is there a section of the cache drive I can assign not to be transferred over to the array? The reason I ask is because I'm interested in having fast loading movie/tv posters and not limiting the fluidity with a traditional HDD. Would I need to purchase one cache drive and one separate drive to hold apps/dockers/plugins?
What's the benefit of BTRFS cache pool other than redundancy? Does a 2x250gb assume 250gb usable, like a RAID1?
Appreciate the suggestions on migrating the data. I am considering purchasing a 6tb but they're extremely pricey. I was lucky enough to get my HGST for $180 but I doubt a deal like that is coming around any time soon. 4TB seems to be the sweet spot right now as they sell at Frys on the occasional sale for $95 after tax (5900rpm but should be fine for this situation, right?). Those plugins also look super useful. I will be using them shortly once more parts come in.
I appreciate the thoughtful response. Thanks again!