You can save money and have a better build if you source the parts locally. I.E. get a Pi4 4GB ram version (2GB will also work but 4GB gives you room for future projects while the larger 8GB is total overkill for anything other than using the pi as a Linux based PC).

Get a small SD card or use an existing one (8GB will suffice) to do the initial boot and flash the EEPROM of the pi to boot from the hard drive. I've included options for a wired or wireless controller. I own both versions and have used the wireless for over a year and can attest to the solid build of it. Both controllers have a very similar feel to the original SNES controller with a tiny bit of "sponginess" in the directional pad you wouldn't notice unless you had an original controller to compare against. IMO this build is superior because of the case and the rock solid controllers. This case is made of aluminum and does a really good job of dissipating heat (The Pi will throttle itself so it can't overheat but it WILL slowdown if it gets too hot. Another and probably the biggest gripe about these custom kits is they violate the license of RetroPie and they are custom so if your a novice you'll get a lot of gruff on the sub for using custom images (see sub rule #3).

One final thought, to setup a really enjoyable retro experience takes a little bit of effort, think of tweaking a new PC and setting it up to be "yours". By building your own pi you'll gain the experience of tinkering and have a lot of satisfaction knowing you did it and have it setup just how you want. There are so many options to truly make RetroPie your own.

BOM:
Pi4 Official power supply Argone One case with power button and good cooling 1TB external HD 8BitDo Wired controller 8BitDo Wireless controller