1. You could probably grow some low-light corals. It's probably a better idea to get some mushrooms and see how you like corals.
2. Get live rock and dry sand.
3. I have a Koralia 1 in my 10g, it seems to work OK.
4. It's not really expensive. It will probably cost you a dollar or less in salt a week for changes.
5. Get some snails, but don't get hermits. Get a mix of snails... astrea, nassarius, cerith. A pair of gobies would be cool, if you could get them. Maybe a shrimp goby and shrimp pair would work for you. Clowns (percula or ocellaris) are a great beginner fish. Damsels apparently get pretty ugly as they age.