Where I work I have helped build a bunch of voice tracking booths for syndicated radio formats, live syndicated show studios, and jingle/production studios and suites.
You can use Auralex or similar acoustic foam panels on the walls to reduce reflections or noise coming through the walls. Adding a drop threshold to the door and tighter foam seals on the door will help with outside noise from the hall.
The HVAC noise will require modification by someone who is familiar with proper duct sizing and installation and with the special acoustic needs of recording studios. The vent and duct sizing and choice of vent and intake grills and the balance of the system were all redone to cut down on the noise in our studios. I leave the HVAC system set to always have the blower fan on so there is no whoosh when the system kicks on and to keep the sound levels fairly constant. Also with using microphones with the proper pattern and a voice processor and properly setting the compressor and limiter you can make most of the outside noise go away. We use Symetrix 528E voice processors in all voice tracking studios and they work great. You need one per microphone channel though.
If you are doing this in a space that is a commercial building you will want to be sure you use UL approved and properly rated materials so the fire marshal does not have an issue and force you to remove it. Also do not block the sprinkler heads with any material. You must keep several feet clearance around them or the fire marshal will write you up during inspection.
Take a look at the items under “Sound Treatment” at Broadcasters General Store. We order a large amount of stuff from BGS for use in our studios. http://bgs.cc/catalog/index.php
We converted a bunch of rooms that were built as regular offices without the normal framing, insulating, ceiling and door treatments into ten radio format voice tracking rooms. We removed the standard ceiling grid tile and replaced it with acoustic studio grid tiles that are a fairly dense fiberglass 2" thick panel with a sound transparent white coating facing into the room. This cut down on HVAC noise from the two units above some of the studios. It is not cheap at about $60 per 2x4 tile.
About a year ago we also removed several hundred square feet of the original Auralex StudioFoam wedge 2x2 acoustic foam tiles in those rooms and replaced them with a custom full wall cover stretched fabric over 2-1/2 inch thick fiberglass acoustic panels. Not cheap but very effective at controlling noise between the rooms.
If you only need room for 1-2 people they do make and sell a isolation booth pod kit that you can assemble inside an existing room. I do not remember who makes them and I know they are not cheap but for some uses they are appropriate.