If you have Plex pass the key for 4k streaming is hardware transcoding. Quick sync with any newer Intel CPU, like 8th Gen and up, is going to hardware transcode 4k fine. Personally I bought an HP chromebox gen 2 and threw Linux on it to run Plex then have VMs and what not on separate small form factor boxes.
I’d bet you could probably run all the services you’re mentioning on something like a single USFF Optiplex but if you want to throw drives into it for storage then the towers would be the way to go.