You have a wonderful opportunity to build a music room that is second to none. The first decision is the exact room dimensions. This matters very much - and there is very much snake out to distract you.
Fortunately, the science has been done, and done in style. The famous Cox, author of my acoustics text, has published a report of his research into room dimensions, the result of a major simulation considering hundreds of thousands of room ratios. It turns out that most are bad. Perhaps a quarter are OK, and a few percent are good.
His research is reprinted on the website of the University of Salford School of Acoustics (UK). When you read the research, you will note that an inch either side of optimal detracts from the resultant sound. Then there is the rigidity of the walls - not to prevent transmission, but to prevent the walls from flexing like 22nd grade woofers. There was an article on this about 4 years ago in Absolute Sound or Stereophile. I used Quietrock 545, backed by firm styrene over concrete or premium 3/4" plywood, and cases on cases of elastomeric glue from Chemlink.
GOOD LUCK on a TREMENDOUS opportunity.