Welcome to server builds!
It looks like it has been a while since you posted and you might be further along in your project, but if you are still planning I highly recommend you check out the NK 6.0 build guide
If you are planning on doing hardware transcoding for Plex or Jellyfin you are going to want to go with the Intel platform as Intel QSV (Quick Sync Video) is the best game in town for transcoding. The current recomendation in terms of bang for your buck, at least if you are in the US, is to get used gear from Intel’s 8th / 9th gen Coffee Lake platform. There are examples in the build guide.
The amount of ram you need will depend on what you are planning to run on the server. If you want to run a lot of VMs the more ram the better, if you are just running a few docker containers to manage your media collection you don’t need a huge amount of ram. I would recommend at least 16GB and 32GB if you have the budget for it.
For storage I would recommend both using some sort of parity, doesn’t necessarly need to be RAID, as well as backups. Parity is important in the event of a drive failure but doesn’t repalce backups for data security.
For a case that is available new the current go-to is the Cooler Master N400. Personlly I like Fractals Define line particularity the Define R5 if you can find a good deal on it used. Both of these cases are large tower cases, so make sure you have room for them. There are smaller cases but you end up paying more for less space especially if you also need them to accommodate a bunch of drives.
For OS I would recommend Unraid. It is a great home server / NAS OS with a very reasonable learning curve and a large helpful community. Unraid has unfortunately switched to a subscription model for all but their most expensive teir which is unfortunate but I think the benefits justify the cost and the “Lifetime” subscription does go on sale from time to time.
Good luck with your build and enjoy your new server. Post back with your build.