[Sk8r]

One of the best investments on a shoestring might be a 24 g aquapod, I think they're called. They're an all-in-one system [look on our sponsors list where they sell equipment, or look in the Used Equipment forum.]
The virtue of this is that not only do you get more water, you get a fairly compact footprint, plus the gear that it takes to keep a marine tank running well. They're a good novice tank, reasonably priced, with size enough to keep several fish and systems good enough to let you keep some hardy corals.