I have been running Plex for several years now, and I have a Plex pass. I have migrated from Mac Mini to Windows Computer, to a WD PR2100 over these years. My users have also grown from just local, to now having 30+ remote users. I only serve 1080p content. I believe I’m starting to have growing pains again. I can successfully transcode maybe 5 simultaneous streams. I have had users let me know that they are experiencing a lot of buffering. On the NAS side I am also running out of space. I have (2) 6TB drives in RAID 1 and I am down to 250gb. I read the posts “transcoding the JDM way”. I’m interested in the HP 290, but not sure how to proceed. Do I need to upgrade the RAM? Should I upgrade the hard drives in the NAS and continue to use it to store my media? Ideally I’d like more bays than 2, but not sure what is the best option to use that would work with this 290. Any concerns with speed of accessing the media across the network? How much hard drive space is required in the 290 for OS and Plex Server? I assume I should be using ubuntu? 
In my opinion, No. Either re-purpose your existing windows comp into an unraid nas(I recommend moving the innards to a larger case like the Rosewill 4500 though) or follow the NK4 or SNAFU guildes.
Unraid is the best option, but the HP290(plex) and the NAS will still be separate devices
Basically, get a couple dense 8-14TB shuccable hard drives for it(with 2-12-14TB drives you could start with 1 12tb drive in the array, move all the data from the 6tbs, preclear and add the 6tbs to the array, then add the last 12tb as parity). You would end up with 24TB storage and 1 parity drive, but you would be able to add another parity, and 25 more hard drives.
Follow the Transcoding the JDM way with the HP290 and profit. Should be able to get 15+ transcodes out of it, and nothing really needs to be added. You install ubuntu on the 290, then add plex and link to network NFS shares from the unraid nas.
The guides look a bit ominous, but really they’re pretty straight forward. If you haven’t heard of spaceinvaderone check out his guides on unraid, some of them are a little out of date, but most are still relevant and informational. One note with SIO, Use linuxserver docker containers NOT binhex (personal preference, but once I made this switch many of my problems went away)
No, you can but you don’t need to. try it as it is first.
No
500GB, but get an ssd hard drive
On the HP290 yes, but that’s in the transcoding guide. On the NAS, you’d be running unraid.
TLDR:
Replace your NAS with Unraid Server, and replace the windows plex server with a quicksync box.
Thanks for the information. Any specific specs for the unraid server? Can it be AMD? I have an old FX 8350 system collecting dust with 16gb of ram.
Yes, it could be… but it’s cheap enough to replace that hardware with a NK 4.0 or 4.1. It’d be faster, more reliable, it’d produce less heat, and draw less power.
So is there any reason I need two systems the NK and the Plex Server? Can I build a NK system and container it to have one system that is the Plex Server and unraid on the other?
You can certainly run a higher powered nas like the SNAFU build & run plex on it as well, but at ~2k required passmark / transcode it’s going to be much more $$ efficient to just use a quicksync box for plex that can easily handle 15 transcodes to give you some room to grow. Also worth noting, if you use all of your cpu on your server for transcodes, then it can’t do much else.