In agreement as far as condensation goes. I have a heavy plastice sleeve that I can fit my 600mm lens in. Once inside, I place it over the furnace duct to speed up the warming process so my camera is not tied up forever. If I’m working in a vehicle, or out of one, I keep it cold inside as long as I’m shooting. Ive found that it I’m done shooting I can crank the heat on and it heats up slow enough that the camera won’t fog up.
Also, you will find problems if you open the door of a warm building or vehicle and try to shoot out. The warm air rushing out will distorting the picture. I also try to shut a vehicle off to keep engine heat from distorting the pictures.
Try not to breathe on your viewfinder as that will fog easily.
Autofocus will slow down when it gets cold but I’ve never heard of anything getting ruined. I’ve worked out to about -30 without any problems.
Keeping yourself warm is a lot bigger problem. If it is below zero you can hardly dress too warm, actually, you probably can’t. Learn to work your buttons with a heavy pair of mittens on. Taking them off will freeze them. I’ve found a handwarmer nice at times.
Happy shooting, Austin