A Game Boy Advance would be your best choice, as it can play Game Boy, Game Boy Color, and Game Boy Advance games. However, when buying these older games online, it's very, very risky. A lot of people fake them and when you get your game, it's not working like it should. And other times, the batteries are dead and they won't save. :( If you have a local gaming store, I would definitely recommend going there because they usually check games before buying them to make sure they're legit and that they still work. As for your Game Boy, definitely go for a Game Boy Advance SP. They have a backlight and make everything look nice and bright and you don't have to use the sunlight to see your game! lol
Anyway though, this isn't really the place to ask. :( If you have more questions, I don't really know where you'd go since this question is split up between so many different topics. But I think I've covered it enough to get you through. If you need more help or have more questions, you can just contact me and I'll get back to you ASAP. Gotta cut this short though cause I have to hurry to class lol. ;( Good luck to you!
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