Only 1 body car 3D model from surfaces for 3D printing

Hello everyone,

I'm new on this platform; I'd like to start to create customized car model, by 3D printing process (hobby & passion);

I have purchased a 3D model from this website about a Ford Escort. I have received different 3d model file format (UV/STEP;IGES);

please, could you help me? I have to create a solid without internal part - I need only the external surface of the car body.

The aim is to create the resin model for only the car model.

I have many problems to manage the surfaces and trasnform them in "only 1 body".

What could be the fastest method?

Please, I need your help. I'm using SolidWorks but I think it is not the best way.

Thank you, Christian