Having a symmetric room definitely helps with soundstage and channel balance, but I don't think the dormer is going to be that big of a problem. With careful application of acoustic treatments, you can balance the reflections off the side walls and get a great presentation.
Since you are not too concerned about leaking sound out of the room, I don't think it's necessary to go crazy with special construction techniques. Basic drywall (perhaps a bit thicker than normal) on 16" spaced studs works quite well.
You're probably going to want to beef up the floor joists (if that's not too difficult). A good solid floor makes a big difference in bass performance. I'd recommend hardwood floor with a high quality wool area rug instead of carpet, since this gives you more flexibility (size and thickness of the rug will alter the acoustics quite a bit), but this is a fair amount more expensive than going with carpet.
The most important thing is to minimize noise from outside. If you don't live on a busy street, the most common cause of this is from HVAC equipment. If your furnace, water heater, etc. are in an adjacent space, you'll want to provide extra sound proofing on that wall (or between floors). With walls, it's relatively easy by using separate studs for the interior and exterior wall such that their is an air gap between the exterior wall and the studs supporting the interior wall, and add insulation in the internal space. I built my last dedicated home theater this way and it was very effective at minimizing sound transfer.
The other thing you need to look at is the venting into the room. Vents are an excellent conduit for noise, and can also generate noise on their own if the airflow is high relative to the ducting/vent size. Using significantly oversize ducting with a large plenum to allow the air to expand before entering the room with oversize ducts helps a lot. Adding some sound absorption material in the plenum will also help.
You'll also want to make sure you incorporate the dedicated mains wiring you'll need. You may decide to move your equipment around or add additional, so install more lines and outlets than you think you'll need. It's MUCH easier to add these when the walls are open.
I'd also recommend consulting with an acoustics specialist to help with selection and placement of acoustic treatments. As I'm sure you're aware, proper treatments make a huge difference in the quality of sound you'll achieve.
In addition to GIK Acoustics (which seems to take forever these days to delivery product), I'd recommend checking out ATS Acoustics. Their prices and quality are similar to GIK, but they are much faster and also willing to do custom sizes for a small upcharge. I've had excellent service from them.