You don't mention your throughput requirements. How fast is your internet connection? Will you be a VPN server and/or VPN client? A reverse proxy? All that adds overhead but really not that much compared to other services. It just changes your requirements as to how many years obsolete your hardware can be.
Generally, whatever desktops or laptops businesses are throwing out in the trash will be more than enough.
If you have a managed network switch or one that can do VLANs, a router on a stick will work fine, especially if your Internet connection isn't more than half the speed of your server's network card. A repurposed laptop is perfect for this, because it has a built-in UPS and console!
One thing I did learn the hard way is that a lot of consumer "smart" devices wrongly assume to be on the same broadcast domain as any servers, clients, or peers they talk to, so even with avahi handling relaying between VLANs, they won't work. It's annoying having to move your dishwasher off the IoT VLAN just to make it work.