a. How critical is it to use such dedicated hardware - could I use a RPI4 with an added usb3-gigabit eth adapter? (I don't need a wired lan, only a wifi one). How much worse would that be compared to the hw you link?
b. Which AP would you recommend? I think the Ubiquity AP lite one is reasonably low cost - are there better options with similar cost?
a. Running everything over a USB3 hub would probably work, you would just be looking at decreased total throughput ( depending on the hub ). Depending on what you're doing ( vlans ) the pi processor might become a bottleneck.
b. Most Wireless APs are mediocre and run some proprietary firmware ( including Ubiquity ). Personally I just got the cheapest one that supported my bandwidth requirements, and blocked if from being able to communicate with the internet ( the one I got would reach out to some AWS compute instance for who knows what ). Other than that they all kind of work the same. I know people who like the Ubiquity one because of it's nice UI.
Could you please link me to the specific AP you picked?
As an aside - this would make a good writeup if you had the time. Theres very little good info on this - at least that I could find.
( BTW, I would not recommend the espressobin if you want gigabit speeds )
I can't find the one I got originally, but any wifi AP works, like amzn.com/B010S6SG3S Just get one for the approximate bandwidth you want to support.