There is a preference in the default tab of the options dialog that says "show HUD" or hide, I forgetat any rate, that is useful when recording.
There was also a film mod a couple of years ago that would disable all your huds with a bind. I have it if you want it.
there's a default keybind to getting rid of the HUD, i remember hitting it once on accident.
to get rid of things like client text, you'll need an addon. i think rtb has several.
F5, IIRC.Assuming you're talking about chat, just set the chat timeout to 0.