"So... I made new texture for Rocket Sentry using Substance Painter. Also I changed Eye of model, because Portal 1's version looks strange and organic and not dangerous (in my opinion).
About denoise. I just needed to change render settings (C4D Octane) from "Directlighting" to "PMC" and adjust "max. samples". This makes picture less noisy. But it will work at only picture, because for animation render will be too long! So I'm going to search programs with denoise function."