Personally, I bought a cheap fanless Chinese (Topton) router (N5105, 4xi225v3) system from AliExpress running OPNsense that's worked flawlessly for the past year or so, but I'd recommend ordering your minipcs through a vendor where you can easily return it (like Amazon) if it's DOA or otherwise doesn't meet your expectations. I'd also skip the small Chinese vendors if warranty support is something you need. Go with a manufacturer that has a local office/support.

Also, depending on what you want your server for, if you want ECC, your minipc options will be extremely limited (a couple Xeon options when I looked). Personally, I went with an desktop Ryzen + ASRock Rack mini-ITX board in a slim case that was still pretty small since I wanted ECC for ZFS.