Hey Shady! I can see that you're going to turn into one of the great haunters. The first lesson to learn is careful planning, allocation of resources, and the acceptance that none of us has the time or money to really do haunting the way we'd like to, lol. With that being said, here's my first thoughts on the questions.
1. I'm not a big plastic fan, I've used it in limited areas, but to me it just doesn't give a wall type feeling. Have you considered bed sheets. They can be stitched along the top edge to mount on a rope or wire, can be painted with stone type patterns, and won't degrade as fast as plastic will, so you can get several years out of them. I've heard of haunters that get them for free from hospitals and nursing homes. Do you or your folks know anyone in the medical field that can ask for you? You might even be able to pick them up at a local good will.
2. Not sure exactly what you have planned here? An outdoor haunted house is a walkthrough. Have you got two seperate things in mind? More info here please. Whether or not you open ahead of time is of course a personal preference, as well as a serious commitment of time, and again resources. Can you leave it up without fear of thieves and vandals over multiple nights? Are you using actors, will they be available multiple nights, can you trust them to show? Weekends only or school nights? What do your folks think?
3. Open sky or roof? If you're planning on being open multiple nights, that means that you'll have to be set up well in advance, and that means more time for the rain, winds and bad weather to come along and ruin everything. An enclosed haunt also tends to make people a little more claustrophobic, which is a good thing if you're planning on scaring them even more, lol.
4. The best advertising is word of mouth, but you won't get that until you've been in operation a few years. Also, is you're haunt going to be of size and quality that it would justify people coming from across town and beyond? Don't get me wrong here, every haunt and display is great in my eyes, but the general public can be harsh. And also, what if they show up by the hundreds or thousands (yes, I've heard of haunters who get thousands), are you set up for crowd control? For example, we don't advertise at all, never have, not even a flyer on a phone pole. But it's grown to the point where we had 500 last year, and I'll be honest with you, our street and haunt could barely stand up to it. We did make the mistake of allowing people to linger and go through as often as they wanted to, but this year we'll be stamping hands. I can't imagine how many more would show if our whole town of 55,000 knew about it.