Is Sleheyron: Story Mode and Battle Arena free?

Yes. Sleheyron: Story Mode and Battle Arena is released under Free fan mod (see the download page) and costs nothing. Neither it nor STAR WARS™: Knights of the Old Republic has any paid content.

What do I need to run Sleheyron: Story Mode and Battle Arena?

STAR WARS™: Knights of the Old Republic, which is itself free. Sleheyron: Story Mode and Battle Arena is an add-on and will not run standalone.

How do I install Sleheyron: Story Mode and Battle Arena?

The PlayBound Launcher installs it in one click from STAR WARS™: Knights of the Old Republic's mods list. You can also install it by hand — full steps are on this page.