Need help of what Nintendo DS to get?

If you don't plan on playing anything other than Pok�mon, anything would be fine. The DS Lite is probably the most cost-effective. And Black/White is no different on a 3DS than on a DS; the "3D" is in the fact that the cites actually look like cites, not a sparse collection of buildings, so you get that feeling of depth, but there's no actual 3D effect like with a 3DS game. Also note that the next Pok�mon game will probably be 3DS-only, so you'll have to upgrade when that comes out, but a DS Lite should suit your needs fine for now.