In my opinion what you should use for prototypes is blank / vague placeholders. The point is to achieve a sense of the gameplay and features without worrying about (polished) assets. It doesn't make sense to me to spend lots of effort first finding and converting something that you need to scrap and start from scratch later. Or to make copyright infringements as you do so.
Prototyping the art is another task and I personally see lots of things taking the wrong track if you use assets from existing games. You as a player and designer have a certain idea of for example what the HL2 physcannon does, how it sounds and how it is animated. You could easily end up repeating the concept even if you later re-make it from scratch. It defeats the purpose of prototyping the art in my opinion so I would personally use something blank instead. Start painting on empty canvas, not on top of something, so to speak.
I don't see any financial parties growing interest towards the project either when they see something that was mix and matched from pieces they are able to recognize as IP of other huge commercial studios.