R4S cases are out of stock there ... 
Here's a forum post for Raspberry Pi 4
It explains the Raspberry Pi 4 setup.
I have a it in a case similar to this one: https://www.amazon.de/dp/B07ZVKN262
The USB3 network card mentioned in the post is a TP-Link UE-300 - which I think you don't need.
In principle, since you have so little bandwidth to deal with, 1Gbps to/from the Pi across all VLANs enough for your use case, and you don't need a second port because of this.
And you need a switch anyway; so you could plug in WAN into your managed switch, and assign that port a VLAN id e.g. 99 and bring it to your Pi that way.
They call this setup "router-on-a-stick".
You can power the raspberry pi over POE from your switch, using a POE splitter like this one I'm using: https://www.amazon.de/dp/B08HS6NDM2
Overall shopping list so far:
- VLAN POE switch (what are you using for your access points today? I'm assuming you have one)
- Pi 4B (ram doesn't matter much, but you may want to get at least 4GiB in case you think of some new use case? Example: https://www.amazon.de/dp/B07TC2BK1X)
- Heatsink case
- Higher endurance MicroSD card like this one: https://www.amazon.de/dp/B07B9KTLJZ
- POE splitter for powering the Pi - if you have a POE switch; some adequate usb-c adapter otherwise.
Do you really need captive portals? Unifi controller that can run on your Pi can work as a captive portal for guests -- but I find them annoying personally - I was thinking you could put a wifi.myhotel.de or something into the SSID and have a simple webpage there served off the Pi or whichever.