Mine freerange in the yard durring the day- and are locked in a shed at night, but I have very little risk of predators. How are you wanting to keep them?? In a permanent pen....or freeranging??? Will they have access to a few acres- or just a house back yard?? A dog kennel can be fine for just two ducks at night- as long as it gives the aprropriate protection from any predators you may have in your area.
Pool, doghouse, stall mats, plastic netting, tarp, feeder, waterer, chain link (I put those slats in mine, so it keeps it draft free, and them safe from our dogs!). That's all I put in mine. $400 well spent!
I live in Salt Lake City, Utah. We have under 1/3 acre. The backyard is fully fenced. We also have chickens in a coop with a fenced "chicken run." There is room in the yard to set up something for the ducks. When winter hits, the ducks will need to be outside (unless it is SUPER cold, then they will come into the garage). We don't have a pond, but plan on using plastic kiddie pools. We also don't want to spend a lot to build them a home.
I wouldn't worry about the ducks in the cold. Mine were fine during our weird -20º cold snap. Just make sure they have fresh straw and enough buddies to snuggle with and they are perfectly suited to cold weather.
(OK, I admit I do worry about them when it gets below 20º, but so far they have been fine in the severe cold)