Supposed to be getting Welsh Harlequins May 25
You have a lot of bases covered already. I'm sure you know that ducklings are very messy and you'll be cleaning bedding and food/ water trays often. Ducklings are harder to care for they chicks but they're worth it. I mix their food with water and make a mash for them to eat because it makes it easier for them, ducklings can't eat well without water which is why their water dishes need a lot of cleaning. They also grow like weeds, you look away for a second and they've doubled in size! You can actually take them outside on nice days for about an hour and let them play and explore. That's how I like to bond with them, I lay in the grass and when they get cold or sleepy them come over and cuddle. While they're outside you can take a pan used for paint rollers and fill it with water when they first start learning to swim, since it has a shallow end, it's safer for them when they first start. Ducklings are great and the bond you can form with them is well worth the work to care for them!
Most people use sand and pea gravel as duck flooring, I only have sand right now because I make they big fluffy nests of hay in the winter and haven't cleared in all out yet. And I've heard of a lot of people using barns to house their ducks, just make sure it's predator proof. I don't keep big groups of ducks, I prefer small groups that I raised from eggs so the bond is stronger, so I just use bottomless an A-frame coop with half of it covered and the other half wire. Most ducks don't roost like chickens and mine don't like having to go up different levels so the one floor coop works for them. I should also warn you that most ducks don't take to coops easily but since it's a barn that shouldn't be much of a problem. The heat pads should be fine. Ducklings need heat for about two weeks then they don't need much but they should still be kept in their brooder until they're pretty feathered out. I think I keep mine in the brooder for about 5 or 6 weeks before they move out to the run full time.
I can understand his confusion! For ventilation cut a hole and attach wire behind it so nothing can get through it. To keep things from digging under we put wire about a foot out from the coop. Make sure it's not chicken wire. Oh yeah! Try and get them a tub and let them take a daily bath it it, or a baby pool if your willing to clean it often, ducks need at least one bath a day so they can oil their feather properly.