lovemymac -- no matter what you do, you're going to need to work with a dealer/installer/designer. I would also highly recommend that you get an acoustician on the job immediately because you're going to want good acoustics and that has to be part of the planning from the beginning. Often, your dealer/installer/designer will have an acoustician with whom he works -- make sure the acoustician is the real deal -- there are a lot of guys out there who will try to save you money and who think they can do the acoustics without an expert -- that's a sure sign you need to keep looking. Interview several home theater dealer/installer/designers in your area. Check their other work. If they haven't built a theater that is up to the standard you want, don't let them learn on your $$$, keep looking. Ask each one what he can build for $100,000 -- 200,000 -- 300,000.
If you are starting from scratch, from a shell, and building a theater, I highly doubt you will be able to do it right for 100,000 or less. Especially with the size screen and room you're contemplating. The acoustic treatments alone could cost you $30,000 or more and they will have to be hidden behind stretched fabric, which is also expensive to install. Having said that, a dealer/installer/designer should be able to help you set a budget or work within a budget you set.
You'll also find that most dealer/installer/designers have a limited list of products with which they work. These are usually a list of gear with which they have worked previously and have found to be reliable. You want to know this, too. Ask each one what brands of projectors they deal and what lines of audio gear.
Frankly, if you don't know this stuff, it is probably better that you find someone you can trust -- because he has installed other theaters that meet your standards -- set a budget together -- and let him do his thing and build your theater for you.
So, start interviewing designer/dealer/installers in your area. No matter what you want to buy, you're going to have to buy it from a local dealer anyway and you don't want to start the project, do not let anyone sheetrock until you've gotten a detailed plan from an acoustician, so locating a designer/dealer/installer and building a team that includes a contractor, your designer/dealer/installer, and your acoustician is your very first step.