1) OLC's GUI mod is not included with GWX 3. You can get it
here. (Scroll down to GUI 1.2.7, which is the latest version)
2) All you need is GWX 3.0 and a clean install of SH3 1.4 to play. However, many people use OLC's Ubermod, which greatly enhances the environment. I believe some elements are included in GWX 3, but not the full package. The only reason I personally don't use it is because it reduces my game to a slideshow, as my machine is getting kinda old. It's a fantastic looking slideshow, but a slideshow nonetheless.