I'd venture to say that you're jumping WAY ahead on the equipment concerns. Equipment is the easiest thing to upgrade. Upgrading the room, not so much. I'd say that you want to spend your money to get the room done properly first.
What are your requirements for using the room? How many seats? Do you want to be able to crank it without disturbing other in the house? Are there concerns about outside noise (HVAC location, traffic, trains, etc). How close are your neighbors? If you look in the dedicated forum, you will see many instances of new builds (or add-ons) in the which the builder/contractor has NO clue about how to build a proper theater room. Which is why they call them "media rooms," because they sure aren't theaters.
You want to do the things that you can't go back and easily redo after the fact. You want to make sure there are enough outlets and lighting circuits. Conduit for wiring, etc. What type of floor is above (carpet, hardwood)? Make sure speaker wiring is run to the proper places (which is HIGHLY likely to be different than where the builder puts them in their package). Mounting the TV above the fireplace is NOT optimal for viewing. If you go the projector route (and personally, I HIGHLY recommend that!) you'll need an outlet and wiring run to the ceiling.
Twelve feet wiide is doable, but tight,. If there is any way to squeeze out another at least 6 inches, it will be a help. Mine was 13' when I started and I ended up at about 12"3. wish I had another foot. But it is what it is.