At the moment, RPi4 is better option. There are some hardware/OS issues with RPi5 and 48k x512 and higher output rates.
Not really, but minimum 2 GB is my recommendation if the price doesn’t become problem.
The ones supported by official RPi kernel are supported if I’ve picked up all correctly.
That page refers to HifiBerry DAC overlay which is supported, but since I have not tested that particular board I don’t have information whether it will work as intended or not.