Is building it from scratch a possibility? In my experience, coops are very expensive and not always ideal for ducks.
I bought my girls a nice big hen coop for £200, but had to cut better vents, make a step etc to make it suitable for them. It was definitely not worth the price and if I could go back I would definitely have built my own!
For two ducks, for overnight housing, I believe the minimum is 4x4ft of space, the bigger the better of course. Straw makes great bedding in the winter. I use dust extracted pine shavings as a base, and then on one side I add a layer of hay or straw depending on the season and temps. My girls need cleaning out every couple days to keep it as dry as possible in there (since I'm currently giving a small amount of water each night which I dislike but the daylight hours are short and the nights are long).
One foot off the ground would be fine, but you'll need a step or a ramp (not steep) for them to get in and out.
Ventilation is really important. People often say 1sqft if Ventilation per bird, but I find that this isn't needed. My two girls have about 1sqft of Ventilation between them from two vents, and that has worked really well. I wouldnt have less than that though, as ducks produce a lot of wet mess and need good ventilation. The important thing is to put the vents at the top of the house (which will hopefully be a decent height), so that there is no chance of wind chill or cold drafts directly on them. It's a balance of well ventilated but not too open. I have my duck house inside my pen which is covered, and the yard is surrounded by trees so it's well sheltered, which also helps to prevent cold winds.
Just a few of my thoughts and learning curves I've learned along the way! Good luck with ducks, they're so much fun and so entertaining to keep, I bet you'll love them!!
