https://support.hpe.com/connect/s/product?language=en_US&cep=on&kmpmoid=4091408
Manuals, Downloads, etc
ATI ES1000 is part of iLO if I remember correctly.
Depends entirely on what you want to do with it. Mostly storage with some containers and a little light virtualization? TrueNAS Scale. Mostly virtualization with less robust storage features? Proxmox.
Proxmox doesn't come with Linux packages needed for a NAS, like Samba and NFS servers, installed out of the box. There's also no web user interface for managing NAS functionality. Since Proxmox is based on Debian, the components needed for a NAS can be installed from Debian repositories, but their configuration and management would be through the command line. If you're considering a virtualized NAS for Proxmox, I suggest looking into Starwind CVM or TrueNAS as a VM option. Both support ZFS.
Unraid, on the other hand, comes prepackaged as a NAS, complete with a web user interface for management. As Unraid is also Linux-based, it offers virtualization capabilities similar to Proxmox, including KVM and VirtIO. A notable difference is in their containerization approach: Proxmox uses Linux Containers (LXC), while Unraid opts for Docker Containers.
If you're looking to use it to run some VMs and containers - Proxmox. If you're going to use it as a NAS - TrueNAS.
If the OP is going to use it as a NAS, OP should get rid of it. Westmere is worthless for a 24/7 server. You'll pay more for power in a year of running it than what a new machine with better performance would cost.
SCO OpenServer
Are you intending on running this as a 24/7 machine?
Keep in mind, these are power hungry processors that don't idle down particularly well that have overall performance of a potato.
You had mentioned euro in your post which tells me that you likely pay quite a lot for your electric. These machines are dumpster fodder except for occasional use as a learning machine, definitely not worth running 24/7. Especially when a Intel box from the last 5 years will out perform it on significantly less power.
I have the same server and i use proxmox.
when I tried to install Debian on hp dl360 g7 with gui, some errors appeared after the login window. but the gui started and worked on proxmox(but extremely slow)