Is Wolfenstein 3D HD free?

Yes. Wolfenstein 3D HD is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor Wolfenstein 3D via ECWolf has any paid content.

What do I need to run Wolfenstein 3D HD?

Wolfenstein 3D via ECWolf, which is itself free. Wolfenstein 3D HD is an add-on and will not run standalone.

How do I install Wolfenstein 3D HD?

The PlayBound Launcher installs it in one click from Wolfenstein 3D via ECWolf's mods list. You can also install it by hand — full steps are on this page.