Slot 2 = native GBA. Emulation < Native.
If you get an EZ4/EZ3in1/M3 Perfect/G6 Lite, you're fine. The max onboard memory (NOR/PSRAM) you would need is 32MB, as that is the size of the largest GBA games.
All of these carts have 32MB of NOR at least.
It would be really nice to have some GBA flashcards in the store. Already got a G6 lite for my micro but I still need one more card as a backup for the future. The G6 memory can easily get corrupted and it's worth nothing if I lose the USB burner.