That is an MK1, it does not have an SD slot in it like the MK2 and MK3.
The difference between the MK2 and the MK3 is that the MK3 has an internal flash memory that lets you flash a rom to it to play at full speed (similar to the GBA flash cart).
If you purchase a DS right now, there's a good chance that it will come with newer firmware that will not allow you to use the Magic Key, however there is a way around this.
Unfortunately it means you need to buy ANOTHER device called a Passme2. You then use this device, programmed for a specific game you own, to flash the DS to an older, hacked version of the firmware which DOES let you use the MagicKey.