Well, I won a contest by doing that .
If you are just going to buy models and textures, you'd need to do it all by one or two companies/freelancers to maintain consistency in style. If you are able to modify free textures and models so that they match a style yourself, you're probably going leaps faster. Even though the models in general have less quality, it may still look good as a whole.

It can be a development course choice too. First building a prototype, then if the project is a success, let a company remake the assets.