Thank you much for the advice folks!
I think I'm going to sell the metal halide setup (I got a great price to begin with, and it's new in the box, so it should not be too much of a problem to get my $ back) and look for a good T5 retrofit. Any suggestions on quantity? I imagine I will have enough for a 6 bulb setup and break even. I do plan to save for a pressurized C02 kit. DIY (sugar and yeast method) seems like, at best, a short term solution for what I would like to achieve.
The idea of starting with emersed plants first is very creative. Thank you. Can other plants handle the transition from emersed to submerged like the Glosso you mentioned, Hoppy?
Now for the last in this series of questions: since this cube was originally designed for a reef application (I switched to a nano for my reef), it has a built in overflow. I have heard that an overflow is counter-productive with C02 applications. My idea was to cap off the bottom (run it sumpless) and use this space to hide the heater, as an intake for a canister, etc. Any advice on this. Right now, I'm waiting for a stand, so I have time to plan.
I do not want a disaster (my tank at school has had its ups and downs, but I have learned much from it. . . .), so I'm reading and getting as much advice as I can. Thank you for all your help, everyone.