@gh4light: As @ms has pointed out, the Mini Appliance will work just fine in the US, provided you buy the right version. Michael is good to work with, so he’ll make sure you have what you need. You might consider springing for the wall mount kit, which takes advantage of the compact form factor. The only downside is the longer-than-normal shipping times from Europe, due to Covid.
I bought two recently, after years of running IPFire on consumer-grade hardware, and I’m not going back. The four built-in Ethernet ports use a chipset that will aid performance, and the built in wireless (if you buy it) will work well, whereas it was always a crapshoot with consumer devices. The hardware selection process has already been done for you and you don’t need to worry about compatibility.
As pointed out, the port on the back is a DB-9 serial port, and you connect to the console using this, in combination with a laptop or PC (or terminal if you want to go old school!). It works quite well, and if you already have a computer nearby, it’ll save you space because you don’t need a separate keyboard and screen. If you don’t, like other networking equipment, you just need a laptop.
Best of luck,