I want to be able to play Pokemon Diamond/Platinum/Pearl, Black/White and Black2/White2. But mainly I want to be able to use some homebrew that would be able to backup and restore saves on my gameboy advance games.

Guessing you have a a phat DS or a DS lite since you said you have gameboy advance games?
An original R4 (clone?) with Wood Firmware should suit your need.
I believe there is a homebrew called "GBA backup tool" that dumps your GBA games, not sure if the save itself is included or if there's an option for it.
I recommend you not to buy a R4 clone. I have one, and the support is equally to 0. It's preferrable to buy a Supercard DSTwo. It's cheaper in the long term, and you can play more games (using emulators, for example).http://www.r4igold3ds.com/ is the site im planning on buying from, can you find one that would be suitable for my needs? (I would prefer if it would be one that is compatible with a 3ds as I'm planning on buying one down the road)
P.S I have a lite.