To disable the hud in Necrovision, press the ~ key to bring up the console.
While in the console, you can type a letter and then press tab to see all console commands that start with that letter (just FYI in case you want to play around with something like the fov).
To disable the hud, type: hudoff
To enable the hud, type: hudon
To bind a key to disable the hud, type: bind f3 hudoff
To bind a key to enable the hud, type: bind f4 hudon
Now when you press f3, the hud will be disabled and when you press f4 it will be enabled. Replace f3 and f4 with the keys of your choice.
Note: Although you can make much of the hud transparent using the menu options, it doesn't remove everything (such as knife and grenade indicators as well as the crosshair).