as long as it's a proper brand like HP or Dell you should be fine finding parts on ebay etc...
If anything dies here i completely strip every single removable part, and keep it sorted in boxes. a lot of this stuff comes handy in places and can be upcycled on next builds.
might work, but not for a lot of throughput.
CPU is not powerful enough, especially if you also dealing with encryption
You have only 1 NIC on these, which you will need to share for external/internal with some vlan config or virtualization (already cuts bandwidth potential in half
a work around for #2 is to use a USB NIC, which will put you in a bad spot with issue #1 again.
Get an Intel NUC, or ChromeBox or something like that. Or you can even run it virtualized inside Proxmox on another machine in your homelab. This is what i do, and it sits with a bunch of LXC's on the same machine (haproxy, UnifiController, CloudflareTunnel). Basically, all my network contollers on the same machine neatly in separate containers. Powered by a super cheap Celeron G3950 and 8GB RAM. food for thought.