Why not just go projector if you're going go that big? Alot cheaper to invest in a projector and screen/DIY screen.
Not necessarily. My projector cost $5k and the screen cost $3k. So that depends what quality your purchasing.
Sure you can buy a projector and a DIY screen for $3500. But its surely not going to be as good of a picture and a lot more work then just throwing up a TV.