Inserting Other Models Into The Scene

Now that the scenery for the town has been completed, I have begun to merge my existing models into the scene. The amount of polys is now around 520, 000 and the number of verts around 380, 000, so moving objects and generally viewing the scene is taking longer as things are slow loading up and generally editing, so time management may be more of an issue now more so than before, due to software limitations.
One problem I have found when merging existing models into a scene is that the textures can sometimes be a problem. As in the screenshot above, I have inserted the barrels that I have made into the scene with textures already applied, yet they do not appear to be there. They appear when I am rendering the scene, but if I decide that I need to change one slightly, I cannot do so without the materials being present in the material editor. Also, as rendering is now taking a few minutes for the scene rather than a few seconds, it is more time efficient that I can actually see the materials on my objects, rather than having to render each time to see the effects.

The tank has now also been merged into the scene, but with no problems viewing the materials on the tank this time, unlike with the barrels. I have been using the orphographic wireframe view to ensure that all of the objects in the scene, including the buildings, are all on the right level, so that things dont sink through the floor while I am animating. Making use of all of the different viewpoints has made positioning and alignment a lot easier, especially using the wireframe, while the perspective view without wireframe shows what my models look like in their entirity.