Screen size and gain selection

I am building a pretty small home theater room and want some suggestions on gain, size and possibly brand of screen. I also had a friend build a theater room and he just used materials he found at home depot, then bordered it with baseboard covered in velvet. It actually looks pretty good but I wanted some experts opinion on doing this and how much I would be compromising.

I am wanting to get the 5020 epson projector. The room is small as I said, 14'8" by 10'5". I will be mounting the projector from the ceiling and after accounting for the size of the projector the throw back will be about 13 feet. The room has no windows and I am painting the room dark so the lighting will be good. After looking at the calculator it seems I need a low gain of .7 to maximize how big my screen can be.

Having 100" diagonal give me prime seating in the 10-12 ft range and the settings is for low ambient light. This tells me it is the right size but if possible I would like to go bigger. When I adjust the range up to 130" nothing changes as far as throw back or lighting suggestion, it is just the recommended seating distance from the screen. It jumps up to a minimum of 13 ft and I will most likely be viewing from 8-13 feet.

1) is recommended seating distance a big deal?
2) are .7 gain screen typically more expensive then 1.0?
3) what is a good suggestion as far as brand for a fixed screen? My target price is around $350 but am willing to budge if it is worth it. I would also like to go cheap if it won't make much of a difference and splurge somewhere else.

So the screens around $300-$400 aren't worth a damn? Do you think I should be shooting for 1.0 gain?

I also wanted to have an experts advice on distance from the side wall. That may be the limiting factor on how big my screen is. I saw some videos on youtube of peoples set up and one guy had an annoying glare on his walls since his screen was so close.

Well since I am getting pretty much everything from scratch I figured I would get a nice projector and budget everything else. Then I can replace things eventually. I'm getting a speaker set up with only an 8 inch sub. Using a receiver I already own. Using a media center computer I made a couple years ago etc. The first thing I will upgrade is my sub. Then probably my receiver and down the road I would then work on a nicer screen if necessary. Most of the things I am upgrading I can recycle throughout the rest of my house. The one thing I can't is the screen, which where I'm having the most trouble committing with a decision.

When you guys say you need a nice screen are you saying a Grand View Fixed Frame Projector screen 120" for $530 is not considered nice?

If I was going to do this right what ballpark cost am I looking at?