Your description of "wrong" seems to be related to one of two things:

1) Your normals may not be facing the correct way.
2) There might be a problem with the alpha channel of your _color texture.

3D Models are composed of many small triangles, as you know. These triangles normally can only be textured on one side (this is mainly the case for the Source engine). Thus certain polygons on the model may have their normals backwards; that is, the texture is facing inwards towards the center of the model rather than towards the outside. The resulting 'face' seen from the outside is invisible. To fix this problem, you can recalculate your normals (either manually or automatically).

Automatically: Select your model, go into edit mode, navigate to the "Normals:" subheading in Mesh Tools toolbar (on the right-- press "t" if you can't see it), click "Recalculate".

Manually: Select a portion of your model in edit mode (such as an affected polygon), navigate to the same menu as above and click "Flip Direction" to manually flip the normals of just the selected faces.

To check to see which faces have inverted normals, you can change the viewport to showing the Textured model instead of the Solid. Check out this picture.[dl.dropbox.com]

In modeling, the alpha channel of your base texture is usually reserved for alpha transparency. This is true for Dota 2. Thus if you want portions of your model to be transparent, you can use this channel to your advantage. If you don't want transparency, simple save your _color.tga texture as a 24bit tga file to remove the alpha channel.

I hope all of this helps. Good luck.

Hey Helljumper!
Thanks a lot for the quick response.
I do not think it has anything to do with the textures, because the error would also appear when i export JUST the .obj to work with it in another program (i have tried sculptris and mudbox so far, and both give me similar results).
In these program some faces rather appear not to be connected to each other, than wrongly textured. The object can not be sculpted on like a solid, 3D object but colapses at the slightest smoothing (or other operation) due to consisting of 2D surfaces in a 3D arrangement.
Not sure if that makes sense to you :D.

I will try the Normal recalcuation and report back :).
Thanks!

Edit: Well flipped normals must have been at least part of my problem. Now i understand why lighting would be so amazingly hard :D.