I built one last year similar to what you describe. It's a great learning process. If I had to do it again I may just look for a good used tank of a similar size. You control everything though so if you need a specific size its a great way to go. I used MDO board instead of plywood its the stuff they make signs out of and it is used in boats as well. It paints up wonderfully. I paid about $80 / sheet. I actually built two one 2x2x4' and one 2x2x7.5'. For me it was an all summer project. But I had lots of fishing trips in there too.
Lee