Can I download another part for free to test print, than the one that is made available for free download on the 3D model’s page?

The author of the 3D model agreed to make part X downloadable for free because, according to him, this is the most difficult and the most important part of the 3D model. You can download part X for free and test print it. The result you will get for this part is valid for all the parts that the 3D model consists of. Just, don't forget to read Printing Details info on the 3D model's page and use them to set your slicing program correctly.