Is Marathon 2 HD Textures & 3D Models free?

Yes. Marathon 2 HD Textures & 3D Models is released under Freeware / Community and costs nothing. Neither it nor alephone has any paid content.

What do I need to run Marathon 2 HD Textures & 3D Models?

alephone, which is itself free. Marathon 2 HD Textures & 3D Models is an add-on and will not run standalone.

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

Roughly 120 MB, on top of the base alephone installation.

Yes. The source is published at github.com/Aleph-One-Marathon/alephone under Freeware / Community.

Marathon 2 HD Textures & 3D Models — Free alephone Mod · PlayBound