The Mirror modifier.The Mirror modifier basically makes a copy of all the mesh data in your object and flips it along its local X-, Y-, or Z-axis, or any combination of those axes. The Mirror modifier also has the cool feature of merging vertices along the center seam of the object so that it looks like one unified piece.
By changing the Merge Limit value, you can adjust how close vertices have to be to this seam in order to be merged.The X, Y, and Z check boxes dictate which axis or axes your object is mirrored along. For most situations, the default setting of just the local X-axis is all you really need.
Enable the Clipping check box. This option takes the vertices that have been merged — as dictated by the Merge Limit value — and locks them to the plane that your mesh is being mirrored across.That is, if you’re mirroring along the X-axis, then any vertices on the YZ plane are constrained to remain on that plane. This feature is great when you’re working on vehicles or characters where you don’t want to accidentally tear a hole along the center of your model while you’re tweaking its shape with the proportional editing (O) enabled. Of course, if you do have to pull a vertex away from the center line, you can temporarily disable this check box.The next check box is labeled Vertex Groups.
Materials not assigning, textures not appearing. When you selected all verts in your object (in 'edit mode') and open the UV/Image Editor window, you should see how its mapped to the texture. If no texture is selected in the UV/Image editor click the Image icon to the right of 'UVs' in the UV/Image editor window, and select. WRT your import settings for Blender, which I missed earlier, if you select 'Keep Vertex Order' instead of Split, and then check the 'Poly Groups' checkbox that appears, Blender will import the item as one object, with the 'surfaces' translated to materials with default settings. You can then add images to the materials and paint on the object.