If they're in .MAX format, the usual advice is to use the trial version of 3DS Max to convert them to OBJ. The only thing that can open .MAX is Max itself. :(

I'm not sure what you mean by "better". Most of the free cars you'll find for download are meant for architectural work, and are quite low poly relative to those at DAZ and in the Marketplace here, which also have the advantage of working features such as doors, wheels and steering.