I m not very good on texturing, so I ask how I can make the logos and text so they are on right place and right size and so on? Something like on that photo. It is a big mystery for me. I watched Squares tutorial about how to use the painting project and texture to the weathering of materials. But I m not even sure am I on the right way.
Have you tried UV mapping your model then you could place the logo exactly where you want it ?
For decaling alpha based images, I often just use projection mapping. It is a lot easier than messing with UVs and you never have to deal with stretching issues because your coordinates do not match.