Definately consider Carada, they are probably the most popular fixed screens on the market. I just assembled my own last week (Criterion series 110"), the build quality went far beyond my expectations, and I am a stickler for quality and detail.
There are really not any options for a masking system that I know of that is not very expensive. There are kind of two ways to do it, you can keep constant image width (CIW) which is the way 16:9 TV's work. You use a 1.78 screen, and when you watch a 2.35 movie, you have the black bars top and bottom. Some hardcore theatre guys or people with wide rooms prefer to run constant image height (CIH) which is a 2.35 screen, and then when you are watching 1.78 content like HDTV, you have the bars on the sides. Generally doing CIH you will not have as large a 1.78 image as if you do CIW, so you have to decide. Is sounds as if in your room you would be best off with a CIW setup.
Screen material depends on how much light is in the room, how far back the projector is from the screen, and whether you prefer brightness, or black levels. I would use the projector central calculator to determine what is best for you, or you can ask the guys at Carada to help if you decide to go that way (they are EXTREMELY helpful). I am using their standard white screen, but the high gain one is generally more popular for anyone with ambient light.
