The new Optoma Greywolf looks very promising for what you want it to do and at a very reasonable price, too... (except it is a pulldown, but you could cut out the fabric and mount it)
100" mdf = heavy and not too friendly as far as hanging/unhanging for each viewing. As far as colour behind the screen, my own personal choice is a flat black. Do you have pictures of the front of your room?
Give David a call at Carada and tell him your situation and he will be more than happy to give you ideas for a screen that will work in your room, the type of background to have and all this on a screen that is very well priced and highly rated.