I have been told that a succesful marine set up needs to be a minimum of 40g to allow the water to stabilise properly,with marines it is water you are keeping not the animals that live in it.Iwould not consider keeping marines unless i had a very large house,with a 75 gallon tank (large 4foot) you can only have 17" of fish in a reef style tank or 35" in a fish only tank.
You also need a lot of equipment,a UV steriliser,protein skimmer and v.large evternal fiter with good flow production are just the basics and if you want corals you must have a pretty powerful lighting system and a Co2 system,a nitrate filter would help no ends as well.