Just remember that sound isolation can be compared to a pail, any leak is trouble. Having thick or double walled pail with a hole still won't hold water. Look for weakest links in the perimeter. For instance no point in doubling drywall if the door or windows leak.
Regarding treatments, don't let your eye or intuition fool you. Research the science behind absorption/diffusion. Recommend GIK here at Audio Circle for products and advice. Their use of high density fiberglass panels are about the most effective absorbers you can find and reasonably priced (I use ten of their 244 panels).
You have the opportunity to develop a first class room.