: This is the most detailed "constructor" where you design the engine and chassis. While not free, you can often find a free demo on Steam .
: A physics-based sandbox where you can build almost any vehicle. It has a huge community of free user-created car downloads on SimplePlanes.com .
If you'd like, I can help you find for a specific game or look for free browser-based car building simulators. Which would you prefer?
: Most of these games allow you to download "Constructor Kits" or pre-built models from community workshops for free once you have the base game.