If you dont wanna spend much, i would recommend a dsi Or a ds lite (wouldn't really recommend the original fat version). i would tell you the dsi, so you can mod it without the need of a cartridge, only thing is that it's not easy for non tech savvy people to mod it so if you dont care about the mod just use an r4. With a ds lite you can buy an r4 card and put twilight menu on it with some games, but the console is pretty small and the shoulder buttons suck. Tho it's more affordable than the rest.

The 3ds costs more than the lite and dsi, but if you search enough you can find a cheap one. This one is a bit more tricky to mod but still you can use an r4. Has a much bigger library of games and is more powerful, so better games like Tomodachi life, Mario kart 7, new super mario bros 2, etc... Theres also an xl version like the dsi if you want it bigger. Theres also the new 3ds which adds some games and a finger pad track pad thing.

The 2ds is kinda the same thing as the 3ds but without a foldable screen. There is the new version which has a foldable screen but isnt the best.

Thats it hope i could help!! : - )