Well, I am fairly new to all of this also. I will try to answer some of the questions.
1) It seems to be as cheap, or cheaper to buy the whole screen(frame and all), as opposed to just buying material.
2) I don't think you will find anyone to argue that Stewart is the best manufacturer, but their prices reflect this. I believe the (StudioTek 130?)is mentioned often.
3) I don't think you want to go to high on the gain, as hotspotting will occur. Do a search for an explanation. Depends partially on how the projector is mounted, floor or ceiling.
4) Once again, if you have the money go for a Stewart, you really can't go wrong. Otherwise Da-Lite makes some popular ones.
5) I myself use a homemade screen out of blackout fabric, it has more to do with money (I don't have any http://www.avsforum.com/ubb/frown.gif
), then anything. I believe they compete favorably with commercial screens. Although when we get a dedicated room built we will probably get a commercial one.
6) Search the board, spend a lot of time reading, and request some samples from manufactures to compare.