You can etch the brass, but it isn't necessary. Clean it with soap and water, let dry, and prime it. Let dry 72 hours, then apply paint. Let dry 24 hours, and apply a varnish.
Either enamels or acrylics will work just fine. Modern acrylics are every bit as good as enamels -- better, in my opinion, because clean-up is easier and there is no health hazard. I use Vallejo Acrylics exclusively now.