Don't forget about any penetrations in your "soundproof shell" as well. This includes all recessed lights, outlets, switches, etc. If you don't seal them all up then sound will "flank" out of the room. Generally it can be rather difficult to deal with these things when you already have a layer of drywall up. You can't easily get to the back of an electrical box to seal it with a putty pad, for instance.
I know that you are on a tight budget, but if you really want to do it right then I would remove the existing drywall and install an isolation clip and channel system ,such as whisperclips, to the walls and ceiling . Then attach two layers of 5/8" drywall with Green Glue between the layers. Build backer boxes for all of your lights and seal the outlets with putty pads. Do some kind of dead vent or soffit muffler to isolate your HVAC and lastly add a heavy solid core door with seals.
This would give you the best sound isolation performance. Now whether you want to undertake that is completely up to you, but I thought I'd throw it out there so that you know the ideal course of action.
Check out http://www.soundproofingcompany.com
for a ton of great information.