Do you already have a DS? If not, you might consider using that money for a notebook and a USB controller with emulators, especially if it's convertible / detachable. Barring that, even your smartphone / tablet will do as Android 5 and above will natively recognise USB controllers. On top of that, you seem to want turn-based games so you would not even need a controller: virtual touchkeys are awful for real-time games because you don't feel the keys under your fingers and keep hitting the wrong ones, but they are just fine for turn-based gameplay.
If you really want to go that route and get a DS, there are some great turn-based games, like the Etrian Odyssey series (merciless dungeon crawlers) or even Fire Emblem Shadow Dragon which is much closer to the game you mentioned.
But again, on a mobile phone, at least get that MyBoy GBA emulator and the two Fire Emblem, Final Fantasy Tactics and maybe even the acceptable LotR The Third Age (shameless Fire Emblem clone). Surely you can test the waters before shelling money on DS games.