OK, this is likely to be a disproportionately long reply and i'm skipping a few bits due to time constraints.
The 'G' versions include iGPU, more cores the better when it comes to RAR/PAR work on a multipurpose server.
4K HDR and Plex is no more or less complex than you choose to make it. Think of the problem as a circle rather than a line, you can tweak any of the variables to get the desired result (eg throw a TR and 128GB into the mix and your poor client choice is irrelevant, but It breaks down as follows:
Assumptions
Transcoding overheads: 2K CPU/1080p 10K CPU/4k
Transcoding = *BAD*
It's free to make appropriate media choices and not free to change client's and servers.
Media
The natural assumption is bigger is better, while that's true to a certain extent in some situations (you don't want to watch something that looks like it was filmed on a potato), in this situation I assure you your significant others will not appreciate buffering because you chose poorly and have a 25GB rip with Atmos audio that can't be viewed remotely with your 2Mbit upload. If you want to choose completely inappropriate audio format/resolution that it's highly unlikely anyone can or will use or be supported by your your client's, then you will need to transcode and devote resources and/or cash to the server side.
Client
Many people seem to think that if it runs Plex, it's fine. Unfortunately it doesn't work like that, a bad client choice means that you can't direct play/stream your content and the server is forced to transcode. Some client's are just poor (PS3 for example though PS4 and XBO aren't exactly without issue). Again this requires you to change either the media type or have the the server resources to transcode.
Storage
Local storage is expensive up front, has ongoing costs and noise/heat, cloud storage is unlimited but has a small monthly fee, the down side is you're limited to your internet connection speed (hint: rent a cheap VPS/root/dedi with decent connectivity from Hetzner and use a CDN). Consider that your storage also needs to be where your server dumps it's transcoded content, will process it's NZB downloads and do RAR/PAR work along with the other services you are likely running (Tautulli, Netdata, Radarr/Sonarr, VPN, torrent client etc.) and on a single mechanical drive, that will likely be a bottleneck before anything else.
Connectivity
Wired connections should be mandatory, anything else will cause you issues at some point (choosing lower bit-rate media can mask a poor network choice), also consider if you chose remote cloud storage then you are WAN limited and that connection may be shared (Sky box' cache content, software uploads, downloads, other people in the household doing 'stuff').
Server
This is usually the area you see the biggest (and most expensive) mistakes made, those mistakes are often exacerbated by previous poor choices. Made a poor media choice and stored it on a cloud storage provider and think wifi is easy? Good luck. If you get the other parts right, then a Pi3 can make an acceptable Plex server. The other common mistakes are old hardware, that 1366 based Xeon from 10 years ago may seem like a beast, but it's basically a power guzzling, noisy room heater when it comes to Plex transcoding. If you are a Plex Pass subscriber then hardware transcoding can be a game changer, intel and Nvidia are the only credible options under Linux, in intel terms, later generations brought improved quality, Nvidia consumer cards are limited to two concurrent streams, Quadro isn't (but £3-400+ for a P2000 is harsh, that said 75w load) - you can use modified drivers under Linux to make consumer grade Nvidia cards to get 20+ streams (see SlothTechTV on YouTube). Also if running a virtualised solution (docker is your friend), passing through the iGPU or in fact any GPU will require a few tweaks.
Here's a curve ball - Hetzner, Myloc, NetCUP, SYS/Kimsufi and many others all provide a range of VPS/root/dedi boxes for very little, combine it with a CDN and GSuite Business and your life can be very much easier for very little outlay each month assuming you have the connection to make use of it. Hetzner Cloud instance starts at €2.99/month and comes with 10Gb connection, G Suite Business is circa £7, combine it with a domain + CDN and you're good to go. If you do choose to go down this route i'd suggest having a look at PlexGuide.com - it's going to make your life a lot easier.