Well actually, if you want to use these engines for a Commercial use, than you will need to purchase the engine, But the tools are already provided, for the most part.
Check out these plugins that were compiled for Maya:Maya Friendly Game Plug-ins!!!!! This will help you in your choices. Personally I would reccommend HL2 due to the fact that you could create Normal maps for your projects which will enhance the visuals and forget the the Physics, But you must have the game to Modify it or add content to it in any way.
You might want to check out this community, Its a Modding community, where Game modifyers, sometimes form together and create games for Existing games and provide them all for free. Thats how the famus counterstrike game was founded, But Valve purchased it from them and liscenced it. Heres the link: Mod DB
Sorry if this didnt help and if you need specific instructions on anything, ill help to my best ability.
Oh, by the way, You might want to check these sites out as good guides to the two games HL2 and Unreal series.
VERC- Valve guides to using the Hammer editor.
Unreal Maya tutorial
Good luck, Sometimes you might have to export your models to OBJ and Impor them into the Unreal Editor(Epic Tool), and Hammer editor(valve tool)