I upgraded my upstairs windows and chose a modern sash window (was expensive) especially for my a/c as it's a rare 240V twin hose unit.
Single hose units will vent through casement windows fine and I don't recommend sealing it as the exhaust will create a negative air pressure in the room. If you seal the window, the air lost out of the exhaust will be replaced by hot air from the house and you'll notice the a/c unit feels useless. Don't extend the hose either, try pick a location below the vent so the unit will make use of the air that comes in from the open vent.