Looking for guidelines to help choose the optimal hardware or opnsense ?

[qarkhs]

Presumably, you already have a fairly good idea what sort of CPU and how much memory you need based on your current N5105 machine. Does it handle the requirements above? If not, there are review sites that benchmark various 11th and 12th gen Intel mobile and embedded CPUs running Opnsense in various configurations. Memory and storage is cheap, especially if you buy and install it yourself, so you may not save much by being economic.

I agree with Patrick that for business users, going with a Decisio or Supermicro makes sense. In the overall scheme of things those products are likely to be seen as good value for money in that context. As a home user I'm not sure I want to spend that sort of money but think the AliExpress route has a fairly substantial risk of being a false economy. Reading accounts here and elsewhere of some the heating and other issues that users sometimes run into doesn't make me want to buy.  I guess some people love tinkering but I just want a reliable box that won't get me into trouble with family members working from home. I'd rather just pay another $100-$200 for improved design, manufacturing and proper quality control. If you buy something from AliExpress I think you are guaranteed to be a guinea pig no matter what you buy because the models and components are constantly changing. See discussion here: https://forum.opnsense.org/index.php?topic=27938.msg139706#msg139706.

You didn't state which product you are interested in. There are probably decent options that would arrive much quicker. The downside with the manufacturers like the ones I mentioned previously are that they are selling 'industrial' PCs  to businesses who want/need reliability. I think for that reason the product cycle is a bit slower. That means if you want a fanless box with the latest and greatest low-wattage CPU, say an Alder Lake N, you have to wait or go with a cheap PRC box from AliExpress or elsewhere.