As for metal textures, i usually start off with a flat gray and then use grunge pattern brushes in photoshop with darker and lighter tones to create a textured effect. Just google "grunge brushes" and you'll find lots. It's of course also a good idea to study the dynamics of light reflected on a metal surface, there are lots of tutorials on this. For metal bolts/spheres specifically there is a technique that i could go deeper into if you want.
As for the shield, it is possible to set materials to two-sided. It will cause the backfacing side to be darker, but this is usually a benefit rather than a problem. If you don't want this, you can copy the whole (flat) shield mesh and move it backwards a bit, and use the "normal" modifier to flip the normals. This would allow you to use a different texture for the back of the shield (i do this for my roman models). I generally advice against modeling the side edges of the shield, it's better to just use planes since this allows you to use alpha textures with great advantage.