Welcome to addictive chicken keeping.
Cold hardy breeds: most heavy feathered breeds such as Wyandottes, Cochin, Orpingtons, Americauna...... Some of these the roosters get big wattles. Rubbing vasoline on those wattles helps keep them from getting frost bit.
Sand in run is good as it helps with drainage and is fun to scratch around in and take dirt bathes.
Chickens also sun bathe so even though the first time you see it and think they just croaked have no fear they are entranced by the sun.
I have a large coop with a sand dirt floor. I use deep litter method and muck mine in the spring then in the fall.
My girls prefer hay in their nest boxes. Yep this birds really can be picky.
Never feed them any strong flavored food scraps like onion or garlic as this will affect the taste of the egg. Beware of sodium in people food as this is bad for the chicken to get too much of.
I use Purina Flock Raiser crumbles as my goofy bunch prefers crumbles. I buy the calcium shells and mix into their food. This ensures the most finicky hen is getting her calcium. I did offer it free choice but always had 1 or 2 that refused to eat it. Now that it is in their food they eat it.
Ventilation is important when designing a coop even cold climate coops need ventilation. If a coop is too closed up the amonia from their droppings can build up and hurt their lungs.
Chicks versus older: well this depends on how soon you want eggs.
With chicks there is no risk of disease coming into the flock.
Adding older birds increases risk of disease depending on what they were exposed to in their last home. Always quarentine older birds for minimum of 30 days before introducing to the flock.
Enjoy your new additions.