Projector and Screen recommendations

Mounting the screen in front of a window would worry me. Are you going to use blackout shades behind it?

No, I can't use blackout shades. Our HDTV sits there with good results. But we do have good heavy plantation blinds on the windows. If a projection system is not feesable I'm willing to get a plasma TV.

Doesn't appear as though a front projector is right for you.

Another recommendation to consider the Mits 4900. I have one and it is a great 1080 projector for not much money than a 720. It is very bright even in low lamp mode where the bulb will last "stated by the manufacturer" 5000 hours. Kirkus has a great recommendation to look at grey screens, especially if you are looking under 100 inches. My screen is about 13 feet away and 100 inches diagonal and the brightness is great with a 1.0 gain screen. There are quite a few grey screens around that gain and they will give you better blacks. IME the two weaknesses of this projector are the poor on board processing (fixed with a good upscaling dvd player), black levels and the slow iris. The black levels do not bother me that much, but the auto iris is really distracting on some movies. Despite its weaknesses it does everything else great and I am still very happy I chose this projector. With that screen size stated above and at that distance the image would be too bright for me in a dark room even on the dimmest setting, but with a little light I am not sure if it would be better. Maybe someone with more knowledge could say if a low gain screen would be better at that size and distance.

I have a setup like the one you are planning to develop: windows behind the screen, Panasonic projector on the ceiling, and the most inexpensive pull down screen (by Elite Screens) that I could get. I was concerned about the screen in particular, but when I installed it, I was surprised by how not bad everything worked.
Even though my room is very bright, it is absolutely beautiful when I watch Roger Waters or Pink Floyd DVD's and I can still see the sunset on the right side of the window (my room is facing West).
If you plan to use your system during the day, the light intensity in your room may be the only thing you need to consider.