I'd suggest thinking about things a bit, and coming up with a need list, a want list, and perhaps an exotic want list. I found that useful in designing spaces (office, house, home theater) in the past.
If you are buying an existing house, you'll have to modify the space. If you are building, particularly if you are doing the design, you'll have more leeway. The former is faster and easier, the latter is more satisfying (I've done both).
A basement works well. However, as mentioned above, you have to make sure it stays dry. When we were planning, I went to a house show, and stopped at one of the boths for a business that did salvage work, Initially my wife thought I was nuts. However, when I told them I didn't have anything that needed salvage and just wanted to know how to avoid using them, they were actually quite helpful. Concerning water, you are more likely to have problems if you have a high water table, if you build on the lower portion of a hill (particularly if you are next to a paved road that drains with your property down hill, and if you don't have adequate drainage off your roof away from your house. Look at the place you are thinking about buying after a heavy rain, if possible. leaking walls and standing water are obviously something to avoid. If the house has a septic system, the local health department should certify it and getting your own perc test isn't a bad idea. Springtime is a good time to see how well a septic field drains. If the property is really fragrant, you might want to keep looking-unless you are really into a really green lawn.
You are probably going to want to consider sound isolation. If you have a couple of small sons, having good sound isolation will let them sleep at night when you use the theater past bedtime, and when they get older and have lots of teenage friends, will let you sleep at night. A cement floor and 2 cement walls is helpful for anchoring frame when planning sound isolation. If possible, you should avoid HVAC ducts that open directly into your theater space and communicate with other rooms. They transmit sound. Any ductwork in the walls or floor above can also transmit sound into and out of the theater space. If it has to be there, use something like insulated flexduct and surrond it with fiberglass insulation. If you are looking at a finished house, an unfinished basement makes it easier to make these sort of changes, if necessary. http://www.soundproofingcompany.com/ and http://www.soundisol...CFY9W7AodGHFM6g both have articles with lots of useful information on this subject. Good sound insulation is likely to require enclosing the space as well-it also helps the acoustics in the space. However, even if the whole area is open, doing some of the above may help.
A basement may also be constructed without windows. If you are planning on a projector, things work better if you control light. That's easier if you have no windows. You also want a high enough ceiling to allow enough space for a screen. There are web sites that have formulas for what size screen to use in a given space, but I used what my preference was instead. I determined my screen size by going to an afternoon performance that wasn't crowded, and pacing out the screen width and distance that I liked an duplicating it. I'm using a 2.4:1 aspect ratio thats 125" in width, an sit about 10' away. That's bigger and closer than the THX recommendations on the web site formulas, but I like it, and nobody has complained yet. Concerning size and throw distance, you need to include space for ventilation for a projector. Different projectors work differnetly in this regard. If the intake or exhaust for the fan is in back, you can't put it flush with the wall. I got around this by mounting the projector in an adjacent room and projecting through an optical glass port. You can also increase your throw (projection) distance by aiming the beam parallel to the screen and reflecting it 90 degrees off a fron silvered mirror. This latter alternative isn't used very often though. However, the dimensions you quoted would probably allow a projector to put an image nearly 10' wide arcross the width of the room, and almost certainly will across the lenth of the room. www.projectorcentral.com has a calculator that can give you a good approximation of what to expect. I found using the projector on a bare cement wall gave me a pretty good idea of where to postion and what screen size I wanted. Don't rely on the manuals you download or the screen calculators completely. Determine the final size of the screen once you can take measurements from the projector's image in the position it will be in after you are done, if you are going to be using it a near full magnification. And, if you build or assemble the screen outside the theater space, make sure you can get it in after you put it together.
One other thing.Once you get going, post pictures. It gives the rest of us more good ideas.