So you want a proxmox cluster then? how much horsepower will you need? If it is only low then the N100’s could work.
For plex hardware encoding is kind of a pain on proxmox. so i would build a nas that acts as a backup destination and run plex in a docker container. (that’s what i’ve done Nutral's HomeLab blog ) That build uses about 30W with a picopsu and 3 16TB hard drives + 2 4TB ssd’s.
Where do you need the high availability, is it in docker containers? Then kubernetes or docker swarm would run better.
In proxmox clusters you need a quorum so at least 3 nodes and then 1 node can stop working and it will keep running. whenever you write to storage it needs to write it to all 3 nodes, so that will need some ethernet bandwidth if you’re writing a lot.
It’s important to know how much cpu power is used, in a mostly idle system the consideration gets a lot different compared to constant cpu usage.
I also have to ask, what do you want to spend on it?