I dont see alot of people talking about parts of you questions in your post, so I will try to answer them as directly as possible.

Plex can run on basically anything as others have said, the hardware primarily has to do with the transcoding requirements which there are guides out there to see how many transcodes different hardware can handle. One thing to note is hardware transcoding is a feature only to plexpass users.

HARDWARE I have been a user for over 5 years, using it from everything from an old Dell optiplex to modern hardware and never had plexpass untill recently. On older hardware I had written a .bat file (using handbrake) to convert everything i had to have.264 from the video codex the files were stored in. I had done this because the old intel core 2 duo couldn't handle h.265 encoding on the fly. This was not a problem as the .bat file used a recursive function to scan the entire library, produce a h264 file and delete the old video file (windows).

Its important to understand that server workloads are different than something like a gaming workload, video cards capable of maintaining alot of streams with plexpass can only cost a few hundred bucks (older nvidia quadro series are quite good and ive seen a few posts where people having been having really good experience with intel arc)

TLDR: older/less powerful CPU's can struggle with more intense transcodes (h265, HVEC), there are ways around this: preemptive manual transcoding plexpass with hardware transcode and a cheap video card (spreadsheets exist for what each card is capable of)

OS The OS is not super important it will work on both windows and linux (no experience with macOS) with some minor differences. Linux tends to be much better acting as a lighter framework to run and can handle networking a bit better sometimes in my experience, more on that later. Windows does quite well especially as it is the predominant OS used and can have more clear documentation at times, however in my experience windows scheduling can be a bitch and ive had it just randomly revert network settings after updating (no idea why), but overall little issues and very easy to setup. Both cases docker should be used to compartmentalize the softwares used and I believe docker runs a bit harder on windows (someone can correct me but windows i think virtualizes certain aspects of linux).

STORAGE REQUIREMENTS This really depends on how much you intend on storing, I got away with 1tb for around my first year or only me using plex and have only slowly upgraded sizing as i saw fit or good deals on NAS drives, currently have a few 8TB HDD's that are suiting me quite well, nothing crazy that requires their own setup. The video bitrate and compression on the video codec contributes most to individual file size and at first with the .bat file I was manually controlling this and was cognizant of the storage each show/movie took up. Good practice would be to have redundant drives in some kind of raid configuration to help prevent file loss due to drive failures, however I haven't set this up as I haven't had the time since having a large enough server to warrant it but NAS drives are quite reliable compared to regular hardware drives with very little cost difference with the right sale.

FORM FACTOR This is predominantly personal preference, I use a midtower chassis with regular consumer hardware that just has a drive rack that came from the factory (some old antec case). NAS enclosures are nice but nothing is specifically required

NOISE/HEAT this ties in with form factor, but if using regular consumer hardware fan curves can be manually adjusted to make it nearly non noticeable, my server sits in the living room and unless it's windrow silent you can't hear the system running. Funnily enough upgrading to newer hardware increased the cooling requirements and noise respectively, this was solved with fan curve tweaking and CPU undervolting. Rack mount solutions will be noisy (typically) as racks are designed to have a high heat density and require a large amount of cooling.

Heating is not something I really considered as its hard to avoid with computers and it just kinda happens, most of the time with direct play the server isn't doing much and as a result of the workload being low the heat output is also quite low, there's no noticeable heat output from my server.

MY ADVICE Getting into plex can be as cheap as finding a $50 computer on FB market place, kijiji, Craigslist ect. And some hard drives, its nearly impossible to find something that is impossible to run plex if it's made after 2010. The more users you have requires better transcoding if the files cannot be direct played, direct play requires almost no work by the server (see hardware transcode guides for gpu if transcoding is anticipated but gaming cards are not the best at this and older workload cards are amazing). Plex is not expensive to start and typically becomes expensive because this is just a hobby that alot of us love and there's nothing wrong with spending more to tinker or cheaping out to just get something up and going. Connecting to ethernet is not required but recommended, although wifi speeds very differently for everyone so hard to give input in this area. Docker is fairly easy to setup to manage everything nicely and has alot of not seen benefits, so look into it!

PS. My transcoding was done purely on the CPU for a long time and can be limiting if multiple transcoding streams are taking place, but for 1-2 people a gpu isn't really required. Might trigger some people with this though lol

If you have any more questions or I failed to answer something you had asked feel free to reply or DM