Look for a bundle like this [1], the only 'buiilding' it requires is putting the Pi into the case, and plugging in the controllers and PSU. It doesn't come with RetroPie, but that's easy to put onto an SD card.

As a bonus, there are also cases like this [2] available.

[1] https://thepihut.com/collections/raspberry-pi-kits-and-bundl...

[2] https://thepihut.com/collections/raspberry-pi-store/products...

It doesn't involve much effort. Transfer image to SD card, put SD card into the Pi and that's pretty much about it on the hardware side.

After that you just have to transfer some game ROMs, set up your controller(s) and you should be good to go. Takes at most 20 minutes to get it all up and running the first time.

https://retropie.org.uk/docs/First-Installation/