You seem to be a bit confused about some of the more basic terminology, so I'll try to explain the differences between those three products.
Basically, there are two types of devices, slot-1 and slot-2. Slot-2 devices like the Supercard Lite require a slot-1 NoPass device like a Passcard 3 in order to boot into DS mode from the GBA slot, something not normally allowed by the firmware. Nearly all slot-1 devices, like the DS-X, can boot into DS mode fine without any other devices necessary.
Some devices like the DS-X use internal memory that is not upgradeable, while others like the Supercard DS use external microSD cards that allow you to upgrade your capacity for a relatively cheap price.
I'd recommend a slot-1 device for you, and I usually recommend ones with external media as I find them to be more versatile. Of these, I'd suggest looking into the following devices: M3 Simply and R4 (same thing, different sticker), CycloDS Evolution, Supercard DS.
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