why get an action replay? isn't it just a cheat code device? can't the m3 do that?
dldi has to do with being homebrew programs on any cart. basically a way to standardize homebrew for compatibility with slot 1 carts through patching. you don't really have to worry about it, i believe the m3 can autopatch now
you could use wifi for backup, but i don't know the details (too much of a hassle for me to set up. search for rudolph's tools, he has various programs for different devices (slot 2, wifi, and flash carts).
you should look up the EZ 3in1, it allows you to play gba, do opera, and backup you stuff. another slot 2 device is sorta redundant in my opinion.