I have gone the DIY soundproofing route, and have the scars to prove it...If you can afford any of the rooms that you posted links for, than you have a good thing going already. The first room's structural design looks fantastic. If you are not an experienced carpenter or have friends who are...HIRE someone. I am assuming you want to play drums in the "new room" and that it is fairly large (at least 12' X 14'). Keeping drums out of your recordings will not be as difficult as keeping your bassier frequencies "in". The drone and/or rumble that you hear from an adjoining room can easily be gated and just as a ball park figure, at -35 db, it is only a problem if the players might be distracted and/or if you plan on recording very softly with acoustic instruments. The low cut on most mixers would eliminate any unmasked frequencies reaching you in any case.
a couple of building points:
- Do not put the walls of your room against the original walls of the room, this is also "decoupling".
- If you can place your final interior layer of meterial (Gypsum - whatever) at a non-90 degree angle this will help prevent standing Waves, phase cancellation, etc,...
- If this is a one-room studio than you can forget about recording live with multiple players unless the room is very large, and you have great monitoring options.
-Placing an acrylic drum booth in the room could make good live recordings possible (simultaneous multiple players).
Good luck to you!