I do not have a SW tank, but I have done extensive research since that is my ultimate goal.(Husband said I should start with
FW first and I am glad I did!)
1. Most SW experts will tell you to start with a 50g or larger because it is easier to control water parameters.
2. I would definately get Live Rock! It acts as a filter, it gives beneficial bacteria, and it looks pretty awesome!
3. The things that grow on the live rock, are you talking about corals and anemones? If so, I would not get those as a beginner. They take more skill, and they are more expensive to start out with because of the lighting requirements.
4. It can be very expensive to start SW. It depends on what grade of equipment you choose. I would reccommend spending a little more in the beginning to save heartache later. You get what you pay for with aquarium equipment in general.
5. I would do a lot of research and reading before you even go to the store to purchase anything. This way, you do not buy anything that you do not need. There are some suggested books on the beginners section of SW in this website, and I would read as many as you can get your hands on!
Good luck, and show pics when you decide to get started!