The short answer to "how do I convert a set of surfaces to a solid object" is to use the Boundary Fill command.
However, this model is kind of a train wreck; it's going to be a long, uphill road. Parts of it are actual zero-thickness surfaces. Other parts are bounded volumes. But the volumes are generally not watertight and the surface normals appear to be random. Faces are sometimes nearly coincident.
The fastest approach would be to rebuild the object. You don't have to start from scratch. Use the existing faces as sketch planes and project in relevant geometry. Build the new version right on top of the old one - Fusion 360 won't mind. For some things, like the wheel wells, you'll want to copy some of the existing faces into a new Patch and then Thicken the patch to make the component a solid.
Time consuming, but not difficult per se. However, you'll need to draw on many different aspects of Fusion 360 to do it efficiently, so this is best approached as a learning project. If you just want it to work, you should probably look for a different tool.
