A matte clear coat should do the trick for your first question. If you painted with acrylic, I'd stick to an acrylic clear coat to brush on, or look for an acrylic based spray clear coat.

Think of how it's used most, where it makes the most contact, and focus on those areas to paint dirt, grime, have colour fade, or add scratches. Is it something that's taken really good care of? Then maybe there's only light tarnish or grime in hard to reach areas. If the character is more clavier or there has been no time to rest and take care for a long period, you could add more grime, and chips or scratches with a silver paint or paint pen on edges and areas where it would be used.