Same here. So I wrote a guide on this a while back: How To Create A NAS Using ZFS and Proxmox (with pictures)
I spent about $500 US on mine. It’s just for tertiary backup so… ~refurb disks, installed Proxmox to a usb flash drive and some cheapo add-in cards. At $1500 AUS, I would recommend new, or at least “in-warranty” disks and an HBM card. CPU/mobo does not matter beyond ECC support unless you want your Plex server to do transcoding too. Then get at least an AMD 6-core CPU or Intel 4-core w/HT. An R5 1400 would also work.
Probably. At $1500 AUS
For cases…
The NAS favorite case is: Fractal Design Node 304 ITX case for $75 + shipping.
I went cheapo and did a Rosewill Stealth ATX mid-tower for $50 on ebay.
There are like… actual 4U server cases designed for rackmounts as well. Newegg search.