I feel like you're speaking a language just out of my reach. I'm almost there, but can't quite close the distance. Why do I need a Windows box and a NAS? Couldn't I use the WIndows box to bring my server online and just have a normal external hard drive connected to that with all of my media on it?
Also, it really doesn't need to be a hardcore setup - as long as it can handle Direct Streaming, I'm happy to pick up the slack and convert all of my media to formats Plex doesn't need to fully transcode.
Truthfully, you don't need a NAS. I've been in media collection/sharing for eons, so I often forget that not everybody has taken my path. Yes, you can go with just a box and be fine.
My advice on storage is to get twice as much as you think you need. That will give you plenty of room to grow. I also often forget how long it took me to fill a 6tb NAS before I had to expand beyond that. I used it for several years of active media gathering.
If you plan ahead of time to direct stream, then you can get away with the most basic and minimal of PCs available on the market. At that point, your pain point will shift from how well the box runs videos, to how well/fast can it boot at all. I would shift your focus to the overall performance of basic use. In my world, I sit comfortably on most i5 processors for basic uses. More than enough to handle basic Plex operations.
I've done a bit more research and feel like a bit more of an idiot. I'm not too worried about my client (which I think is what you're talking about in regards to the Windows box), because I'm planning on running Plex through my phone, computer, PS4 or Roku, whichever is closer really.
These devices are "clients." The Windows box would be considered the "server." Plex comes in two pieces, the Server which handles and processes the files, and the Client, which is the app on the viewing end. The Server will take the file in hand and communicate with the Client to determine what types of files can direct stream. If the file doesn't match what the client takes natively, it will transcode. This is where you run into issues when you want to run multiple clients. You have to narrow down the single file type that will work on all of them if you truly want to avoid transcoding. It's way easier to get a processor that can just do the job.
All I'm trying to figure out is my server solution. What's the best server to run a single 1080p Direct Stream, on a budget? If I can get full 1080p transcoding going on three devices at once for under $200, that'd be great, but I am trying to be barebones and realistic. It'll give me something to build up to.
I wish I could just spit out a link to a box that could do that, but as you know, the sea of options is mind boggling. I point back to the Passmark rule of thumb. If you want to handle 3 simultaneous 1080p streams, then you'll want to look up a processor that scores around 6,000. Find a box that looks interesting and Google the score of the processor.
Bonus points if the processor supports Intel QuickSync, (most modern Intel "i" processors do) cause then you can get into Hardware Acceleration, which takes tons of workload off the processor. Again, a Google search will answer that question for the specific processor.
Sorry if all of this seems overwhelming. I'll gladly clarify any questions you have. This seems tough because you want to get it right... and that's what we're all here to help with.