Anything under a ten gallon is difficult to find equitement for. I have a ten and a fifteen tall. You may want to look at a delux cube. Make sure it has good power compacts on it and a built in skimmer, or room for one.
The bigger the better for marine aquariums. This is so if you make a mistake at anytime, a smaller tank would die or fall ill alot qiuicker than a big tank. And once you get started you want to keep adding more and more and more....