Two good advices:
1.(mandatory)read the reamdme/license/terms of use .doc or .txt thats included in 90% of what you can download - as long its from any official FPS-related site.
2.(optional) Don't use anything thats not free for commercial use that needs explict permission/crediting etc. Its hard enough developing a game without thinking about copyroght issues.
PS: The same goes for sound, music etc. Only use stuff thats free for commercial use, there is enough available on the web.
In case you find my grammar and spelling weird ---> native German speaker ^^