For my larger boxes, I only use SuperMicro. Most other vendors do weird shit to their back planes that make them incompatible or charge for licenses for their ipmi/drac/lightsout . Any reputable reseller of server gear will offer SuperMicro.
The disk to ram ratio is niche, and I've almost never run into that outside of large data warehouse or database systems (not what we're doing here). Most of my machines run nearly idle even serving files several active streams or 3gb/sec data moves on only 16gb RAM. I use CPU being maxed out as a good warning that one of my disks needs checking, since silvering or degraded in ZFS chews CPU.
That said, hypervisors eat RAM. Whatever machine you might want to perform torrents, transcoding, etc, give that box RAM and either a good supported GPU or a recent Intel quicksync chip.
For organizing over the arrays, I use raided SSD for the downloads, with the torrent client moving to the destination host for seeding on completion.
Single instance of radarr and sonarr, instead I update the root folder for "new" content any time I need to point to a new machine. I just have to keep the current new media destination in sync between the Arr and the torrent client for that category.
The Arr stacks have gotten really good lately with path management, you just need to ensure the mounts available to them are set correct.
In the event I need to move content between 2x different boxes, I pause the seed and use rsync to duplicate the torrent files. Change path and recheck the torrent. Once that's good I either nuke and reimport in the Arr, or lately I've been doing better naming convention on the hosts so I can use preserving hardlinks. Beware, this is pretty complex route unless you are very comfortable in Linux and rsync!
I'm using OMV on bare metal personally. My proxmox doesn't even have OMV, it's on a mini PC for transcoding. I see no problem running OMV inside proxmox though. My baremetal boxes are dedicated for just NAS duties.
For what it's worth, keep tasks as minimal and simple as you can. Complexity where it's not needed can be pain later. My nas machines are largely identical in base config, with only the machine name and storage pool name different.
If you don't need a full hypervisor, I'd skip it. Docker has gotten great in its abilities. The easiest docker box I have was just Ubuntu with DockGE. It keeps it's configs in a reliable path so easy to backup your configs etc.
Alas, I'm the sysadmin. It's not domain joined.
Microsoft is forcing it, but many configurations are delayed due to the critical issues they've been hitting.