I use both Proxmox and libvirt on Debian. I don’t use clustering. For me, the biggest advantages of PVE are:

• Good VNC and serial console support integrated into the web interface. (Could probably get something similar with libvirt using Guacamole, but PVE makes it super easy).
• Good VM snapshot management. I’ve found libvirt snapshot management to be pretty limited and/or buggy, and I’ve had to resort to directly operating on qcow2 files.

On the other hand, there are a couple things I like more about libvirt:

• Good support for SR-IOV NICs. libvirt lets you create a pool of VFs and automatically assign a free VF to a VM. (It’s a little surprising to me that PVE doesn’t do better in this area.)
• Simpler with fewer moving parts to break.

I use libvirt for my most critical VMs (network infra like router, DNS, and DHCP). I strongly prefer PVE for anything where I’m going to be interacting with VMs regularly, like testing or lab setups.

If CPU speed is not particularly important, I suggest getting some Wyse 5060 thin clients. You can get these for around $30 per unit on eBay.

Then upgrade the storage and RAM. They can take up to 2x 8GB DDR3 SODIMMs. Storage is a SATA DOM. You can just remove the board from a lot of 2.5" SSDs and use this. I’ve done this with a 500GB Samsung EVO 850 and it was pretty easy.