GameStop is a great place to buy retro gear because you can easily return it to a store if it doesn’t work. I don’t buy retro from them often, but it’s been very convenient when the retro stuff I got was busted, which is pretty common for disc based stuff.
For a GameCube, I recommend you look into getting a Wii. The first editions of the console can play GameCube games and have ports for the controllers and memory cards. The advantage of the Wii is that you can save a ton of money on a component cable, allowing you to play on a HDTV. The GameCube component cables cost over $300.
The downside is that you lose the Game Boy Player. If you’re really set on a GameCube, you can get one HDMI modded, but that takes some time. There will eventually be an external HDMI adapter too, but it’s still in production.