The HW assembly - putting the board in the enclosure with four screws and plugging in a microSD card and a PS - was trivial. I used my PC and a SD-to-microSD adaptor to copy the OS image (and then plugged the microSD into the RPi). No RPi-attached monitor is needed, only some run-rime web-interface based configuration. RoPieee page offers installation instructions.
Personally, I use a regular RoPieee SW as a Roon end-point (with an external USB DAC). The RoPieeeXL adds some other functions - including Apple Airplay and DLNA client (though I have no experience with the latter).
RPi-4’s internal WiFi is good enough, so no external dongle is generally needed.
If you buy two SD cards can you install one with RoPieeeXL and the other with MoOde OS and see which one you like best? Is it as easy as installing an OS on each card and then swapping them into the RPi4 for testing?