Hi Ariane,

The only face style that allows complete "shadelessness" is hidden line mode but that comes without textures. So with a textured model, you will always end up with some kind of shading on the faces.

Imported files can also be "fixed" - there's just a couple of things to take care:

when importing, check out the import options and make sure NOT to preserve drawing origin. While there, make sure to import with the correct scales finally if the imported group has its origin somewhere etremely out - it can happen if there is some reference geometry there in the original CAD file - delete that geometry, explode the group and while everything is selected, regroup again.
Always a good way to use Zoom extents (Shift+Z) to see how "large" your model is. If you seem to zoom out too far away, there is some problematic geometry somewhere. Press Ctrl+A to select all and see what is highlighted blue (beside your model) to identify the culprit.