I'll leave the UG filter to someone else (lots of people dislike them as they tend to get plugged/slowed right down, they compact the sand, and they can become a nutrient sink). A skimmer isn't necessary but it can increase the amount of bio load that your tank can handle (skimmers take out a ton of crap). I prefer sand for a SW
tank for the look and for small critters to live in, also some fish need sand to burrow in. You can and should have rock, live rock, dry rock, DIY rock, it's up to you but you should have some rock. As far as the fish go I would just do some browsing and see what I like. A place I like to check out fish is aquacon.com
they have a pretty good selection, some good basic information and the site is fast so it's easy to browse the pictures.