Is Tux Racer 3000 Kart Model free?

Yes. Tux Racer 3000 Kart Model is released under CC-BY-SA-4.0 and costs nothing. Neither it nor SuperTuxKart has any paid content.

What do I need to run Tux Racer 3000 Kart Model?

SuperTuxKart, which is itself free. Tux Racer 3000 Kart Model is an add-on and will not run standalone.

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

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

Yes. The source is published at github.com/supertuxkart/stk-addons under CC-BY-SA-4.0.

Tux Racer 3000 Kart Model — Free SuperTuxKart Mod · PlayBound