"I've got plenty of shorts but no silk suits" - gee, I wonder where THAT reference came from :=)
Doors can be a real problem, sometimes moreso than windows. A heavy fire door can give a fair amount of isolation, but if it includes an air space inside, then you won't gain much by adding a second door later if needed. A solid core exterior door is several dB better than a hollow core (nearly useless) - the same physics apply to doors as walls - hermetic seal, double leaf, widest possible air gap, maximum mass, preferably DIFFERENT mass per leaf if using double door...
Sealing is one of the big problems. If you're willing/able to live with stepping OVER a threshold, you can put soft foam weatherstrip on one edge of a 1 x 2, and just caulk/screw it down across the door bottom gap so that the door slightly compresses the foam when closed. This can be done by closing the door, then pushing the piece of wood up against the door bottom firmly, and drilling the screw holes - then, mark where the piece goes by running a pencil line along the edge, open the door, put a bead of caulk about 1/2" in from each edge, and screw it down. You can use the same method around the other three sides of the door.
If you can swing it, and need your doors to be as good as the walls, you can do double doors and double wall frames, as long as each assembly has only one center of mass and they are separated by nothing solid, only insulation, air, or at most neoprene in limited spots (for sway brackets)
The alternative I'm aware of is several hundred $ door bottom kits from places like
http://www.acousticalsolutions.com/products/doors/seal_kits.asp
Or several thousand $$$ complete door assemblies from these guys or similar -
http://www.overly.com/doorCo/Products/Acoustical/index.cfm
Depending on your floor plan, you might be able to get double duty from an airlock - it could be a fair vox booth and a good isolator between two other rooms... Steve