When it comes to fabrics, I often go here for guidelines http://pixiekitty.net/fabric101.htm
I hope that could help
I've heard that Ben Nye makeup products are pretty good. That being said, I have never actually used them myself. If you go through the first few pages of this forum, it seems that people have reviewed some body paints so that may help as well http://www.cosplay.com/forumdisplay.php?f=247
As far as the gun goes, I know that cons have a lot of rules when it comes to weapons. If you're going to one, make sure you read the rules/safety thread before you get to painting. In most cases, they won't accept guns if you paint over the orange tip. They'll also end up zip-tying the trigger since the sound can usually frighten people.