I've used a window projection for the past two seasons and it's been the highlight of the night each time. Be prepared for people to ask you a ton of questions about it!
I used Ghostly Apparitions first and then last year I used
Zombie Invasion. The zombies were by far the biggest hit. I had people lined up in cars taking pictures and filming video. It was great fun!
I used a cheap white plastic table cloth that I got at Dollar Tree for my projection material. It worked well for the zombie projection because the video actually looks like they're behind a plastic sheet to begin with, so clarity didn't need to be spot on. Plus, I'm using an ancient projector that I got from work (they were throwing it out), so it can't give me a clear projection anyway. My projector was probably 5-6 feet from the window in order to get the proportions right.
I think it's worth spending some extra money to get a decent projector because once you've done this type of illusion, you will probably find yourself trying to figure out other ways to project images throughout your haunt!