I tried Authelia but couldn’t set it up, so I’ve been using Authentik and have been quite happy. The only downside is that I had to configure it using the GUI instead of with config files, which I think would have been a point for Authelia, but couldn’t get it to work properly.

When I started my home server was an old laptop, eventually it became an old desktop, and now it’s server specific hardware. My recommendation is use whatever you have at hand unless you have specific reasons. I went from laptop to desktop because I needed more disk space, and went to specialized hardware for practical reasons (less space, less electricity, easily accessible hot swappable hard drives). But for most of the stuff I have there an old laptop would still be enough, heck, a raspberry pi would be enough for most of it.