Is Unlimited Armament Works free?

Yes. Unlimited Armament Works is released under Open Source and costs nothing. Neither it nor Mindustry has any paid content.

What do I need to run Unlimited Armament Works?

Mindustry, which is itself free. Unlimited Armament Works is an add-on and will not run standalone.

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

Roughly 8 MB, on top of the base Mindustry installation.

Yes. The source is published at github.com/Eschatologue/Unlimited-Armament-Works under Open Source.

Unlimited Armament Works — Free Mindustry Mod · PlayBound