EDIT: Also utajoker - having 400L marine tank is impressive but this will cost you a fortune and this is not only for live rock, you need protein skimmers and other equipment in a tank this size but this all depends if you want fish only tank or reef tank which is even more expensive. You can get a lot of information from lets say ultimatereef.net which is a dedicated marine tank forum (probably the biggest in UK).
i have all the equipment, spent ages researching all about it spent a small fortune on ebay then came to setting up and buying live rock but ran out of cash.