In my wife's class, I set up a 33 Flat back Hex with a Clown Pleco
, 10 Pygmy Cories, 15 Neons, and a male and two female Red Wag Platies. I wen't with fish from Spanish Speaking countries because it's a Spanish Class.
What kind of Class do you have? How much space would you want to give up to a tank? Does the heat/AC stay on all winter/summer? If not, what's the most extreme ambient temperatures you would get in your room? Do you have a room that you'll be keeping a while, or do you get moved every year or two?
Ideally you want a tank with fish that will live nicely on prepared foods, that's big enough to be interesting at a distance, but small enough to move without heroic efforts. Also, the bigger the tank, the easier it will be to keep the temperature and water parameters more or less stable.
Another thing, how far are you from a sink, and is it a sink that you can turn on and leave on, or one of the ones you have to hit a button to get 2 minutes of water flow?
All things you need to consider.
And how much would you want to spend? You can probably get a lot by way of donations or craigslist, but still, some things are going to cost.