I have settled with Substance Painter as my external texturing application. I am sure there are other alternatives, but I was impressed with the ease of obtaining nice results, especially in order to dirty up textures and such.
I am currently in the process of preparing a plugin to ease the mapping of Substance Painter generated textures to C4D materials. Might take a while to finish this, but my first attempt was already helpful in mapping basecolor, AO, roughness, height, normal, metallic and ior into a R17 reflectance material.
I am still using Substance Painter version 1.7.3 and have not yet upgraded to the newly released version 2, but will do so very shortly.
While writing this, I notice that Vozzz replied, commenting on the use of 3D Coat. I've tried that one as well, but in the end preferred to purchase Substance Painter instead.
No sss just a simple material with some basic stuff in the reflectance channel I did a slight colour correction in photoshop. Managed to make quite a nice set o plastic materials, if you want any let me no