At least two.. i only have Muscovy, they do roost lol but that is there trait.. they just get pools and while they are less water loving than most ducks a lot of people use kiddy pools, easy to keep clean and you control where they are... less danger too..(snappers! come to mind)
Anyhow, i love calls.. but they are super teeny.. and loud so those things would have to be considered... I free range my 'scovies and lock them up at night.. but there barn contains pretty much what a chicken coop would... ventilation, windows and is secure to keep predators out and again because i have scovies i have a shelf/roost in mine so the girls can be up at night if they choose, i also keep my drakes separated from my ducks at night.
For feed i use a flock raiser by purina since i have birds ranging right now from day old through to over 1yo and offer free choice oyster shell for extra calcium for the ducks, plus they forage.
ETa; how old is this sister? personally unless she is an older child i would narrow down the choices to a couple and have her choose from that, you have to decide on a breed that best suits not just cute which is what a child would focus on but then all ducklings are cute!