Hi Poppy,
Please don't get put off by the answers people are giving. I know it is brutally honest. When I first enquired at Hollywood Fish Farm, I got or I felt like they shot me down on a firing line!
But I perservered and did A LOT OF READING on this subject. Online and in books.
What helped me the most to understand the lingo and what I really needed was a book called "The New Marine Aquarium: Step-By-Step Setup & Stocking Guide" by Michael S Paletta. I based my tank on what was in the book, with some modifications. A lot of people on this forum have large tanks around your size and bigger, that's why it costs so much more as you need equipment that would cope with the amount of water you will have. To a certain extent the smaller the tank the cheaper, but in saying that, my 126 litre tank (including my sump) has cost me just over $2000.00. I have not bought the most expensive equipment, nor the cheapest. The most expensive equipment would be the lighting, then the protein skimmer then the pumps (yes "Pumps" plural).
I highly recommend getting the book above. I think it is a great starter for beginners, then after reading, go to the different forums and do searches on how to begin a new marine tank.
Amazon has this book at a good price. That's where I got it from...
http://www.amazon.com/gp/product/1890087521/102-2774091-8439304?%5Fencoding=UTF8&v=glance&n=283155
Good luck and just take it slow, one step at a time, and get the tank done right the FIRST TIME.