Check the numbers carefully, i’ve known people spend a lot to save an insignificantly small amount of power per day because they didn’t measure actual power usage with a meter and assumed peak TDP equites actual usage. The other issue is in the case of intel powergating has been about since the 2xxx series and literally uses bugger all power when the cores aren’t active, it can be very efficient at performing CPU heavy tasks then switch the additional cores off again. A less powerful CPU would potentially still be at 100% for much longer.
That said NUC’s are great little devices (I have 3), but each and every single one of them is now CPU limited to the point that I have to be creative/use something else to do the heavy lifting, same with the Microservers I own (N36-N54L) as all are soldered CPU’s and sooner rather than later, they won’t support a required instruction set/standard and lack the raw computational power to compensate - at least with them a GPU upgrade can add modern hardware transcoding support.
As to how low you can go, that is going to be down to your choice of clients, media type/encoding, resolution and connection/bandwidth to the server.
If your client’s can direct play/stream without bandwidth constraints, then your server requirements are usually very minimal, it literally just needs the minimum of CPU power to enable it to send the media stream. Unfortunately in the real world, not everything can natively play every audio stream, you either need to be selective, or as a lot of the ‘known’ brands refused to licence the appropriate codecs, randomly the ‘no name’ stuff often does, that requires the server to transcode, things like (non hard coded) subs require the server to transcofe, then you have users, specifically the ones who have inappropriate connections and unsuitable hardware and think they’ll play 4K content over 2mbit, they will force a server to transcode.
Now consider the ‘other’ tasks you mention, downloading for example, torrent’s are pretty undemanding other than checking the download is 100% on completion and moving your files, but they’re slow and obviously you’d run a VPN connection for privacy, most modern CPU’s support hardware encryption, but encryption standards evolve through necessity. Newsgroup downloads tend to be heavy as they’re fast and RAR/PAR work is very CPU/IO heavy, this can impact on other services. Then things like Radarr/Sonarr will presumably be running to index/process content because who does it manually? Finally you need storage space, externals are fine up to a point, but you don’t want to be dealing with multiple external USB drives and media everywhere, spaghetti = bad and NUC’s are quite limited in terms of internal storage.
Right now I have a first gen i3 NUC (3127u/8GB/480GB SSD) doing all of the above and uploading the end results to cloud based storage. It’s only serving Plex to local clients, the majority of which will direct play and it copes fine for 720/1080. Throw an audio transcode and some subs in and it can get a messy, even with RAR/PAR post processing reigned in. The point I’m trying to get across is that expensive, cute little box that supposed what you want right now, that’s going to be a limiting factor in a few years. If you go SFF, your upgrade options will be much easier, likely cheaper and certainly more plentiful.
If it were me, i’d look at an old G8 microserver, you have decent CPU upgrade options, good power/thermal efficiency and the ability to add 4-6 local drives, combine it with Unraid and you have a very flexible box, if not then price up an equivalent SFF build.
