Based on my fairly recent research, 1/4 inch outer diameter seems to be the hose/airline size of choice, and the polyethelyne is cheaper than nylon or polyurethane and you can get it at Lowes or Home Depot on a moment's notice.
I haven't found a good discussion as the benefits of 12vdc, 24vdc, or 110vac yet. I think it's a matter of personal choice and what you use throughout your haunt. I guess that one benefit of 110vac is that you can pull a powercord off of any failed appliance around the house for free and hook it up to the solenoid, whereas 12vdc and 24vdc require the purchase of a power supply or a wall-wart.
At first, I thought that some of the prop controllers would output 12vdc to the solenoid, so you wouldn't need an extra power supply, but the more closely I look, that doesn't seem to be the case; just relay controllers.
Best of luck with your excursion into pneumatics. I'm already a few hundred dollars into it (several cylinders, several controllers, several solenoids, several fittings) without a proper pneumatic prop to show for it yet, but I'm on my way.