You do not need to buy a memory card and honestly, the people telling you to buy a charging cable are being a little silly too IMO. You charge it in the dock for the most part. Everything you need is in the box except a screen protector. I would get that.
If you want to throw in something else, get them a $10 or $20 card to buy some games from the eShop instead of the extra charging cable.
I buy most of the big games on cart. I also buy eShop stuff, indies and arcade games. I still haven’t needed to upgrade the storage. Downloads I own include… Hollow Knight, Enter the Gungeon, Capcom Beat 'em Up Collection, Space Dave!, Golf Story, Neo Geo titles Last Resort and Top Hunter, ACA Donkey Kong, ACA Excitebike, Kamiko, Gunbird and probably a few more I’m forgetting. I also have Fortnite installed and Pokemon Quest, too, both free to play.