Stop. Pretty much everything in your op is wrong and this will end badly.
Lets start with the basic's:
1) Old Xeon's suck, anything pre LGA2011 is dead to you, the e3 12xx v1 and v2 is OK, but it's no different to a desktop chip of the same vintage in this scenario. People have this idea that buying an ancient server/server hardware cheap from eBay is a good idea because it's got Xeon's in it and it's enterprise class hardware, those people end up hearing impaired and financially worse off every month because they ignore the noise/power usage and the noise. DO NOT BUY A DINOSERV! A modern i3 or similar is perfectly adequate and the iGPU is capable of multiple hardware transcodes/decodes which makes life a lot easier, it also won't destroy your ears or make your room stupidly hot. What you are suggesting is rated for 12k of CPU mark per CPU, you have two remote users, your remote users are not streaming 4K and your local clients will direct play, so why are you going this route?
2) You really don't want to transcode 4K. You really, really don’t want to transcode 4K HDR. It's inefficient and you would be better off carrying a split library set-up for 4K (which you ensure can only be direct played using TauTulli) and Plex can only output to h264, it can’t do tone mapping from HDR to SDR so the results are rubbish. Plex suggest 2K of CPU mark per 1080 AV transcode, that's slight overkill and due to the way plex transcodes you could get away with more streams (it goes flat out to fill the buffer to the set time limit and then stops).
3) With a 20Mbit upload and wanting to output to remote users at 10Mbit/1080 then that's a maximum of two uploads, if you have a recent 9/10 series Nvidia GPU then this falls nicely into that capability (more than two streams requires a Quadro or unlocked drivers on Ubuntu), with 20Mbit you don't need a Quadro as everything local should be direct playing or you're doing it wrong and you still have software (CPU based) transcoding to fall back on for subs/audio etc.
With proper media and client selection I have run a Pi3 as a Plex server (no transcoding), it worked OK. The key is to know what you're doing and make sure your media direct plays on your clients.
Now we've got the basic stuff out of the way, how comfortable are you with non windows OS'?
Option 1: UnRaid
You can throw random drives at it, it uses the largest for parity, add plex docker and the excellent BinHex torrent dockers with VPN/Privoxy included. Understand that 2 remote users + torrenting on 20Mbit will suck based on what you have already said you want to do (2*10Mbit/1080).
Option 2: Ubuntu + PlexGuide installer
Unlimited remote storage with a local drive for meta data and transcoding. Your remote storage bill will cost £6-7 a month, likely less than powering the two CPU's you were planning on running 24/7.
For hardware transcoding you need a plexpass (lifetime is 25% off now), if you don't have PlexPass then you need CPU power to transcode. Hardware wise a 6th/7th gen i3 or i5 is where i'd look at, I built an i5 6500 for testing a while back for under £200 all in, my 2630L wasn't much more, parts sourced from here and from eBay, even a 3rd/4th gen is going to do an OK job with an iGPU, but the quality/codec support improved from early generations, 4th onwards is quite capable. 8GB is adequate, with unraid use an SSD cache drive as direct to the array is inefficient and VM's will be faster if using SSD, with Plexguide use an SSD where possible, especially if you want to run Sonarr/Radarr/NZBGet/torrents and transcode at the same time. The other options is to rent a remote server, SYS have a £30/m deal with 250Mbit each way that would fit the bill, Hetzner Cloud do VM's from £3ish upwards, 10G shared each way.
If you have questions, ask. Too many people make really crappy hardware choices and then take a financial hit trying to fix them or go deaf in the process. Don't be that person.
