When computers first came out the software was super cheap and the hardware was expensive. Nowadays the software is super expensive and the hardware is very reasonably priced. However, we now have FREE software. Like the web browser you use there are lots very good yet completely free to use software available. Free to use even for commercial use.
For doing organic looking designs like sculptures and stuff, try
Sculptris or
Meshmixer. Commercial equivalents are Zbrush, Mudbox and 3d-Coat.
In addition to those already mentioned there are a few more you might have a look at. Like
TrueSpace 7.6 and
Hexagon. Also,
Trimble Sketchup, formally Google Sketchup, can do most of the things the others can do if you install all the proper plugins for it. It has a very good 3D simulation plugin and compared to all the others I'd say it is the easiest to learn.
If you want to do engineering stuff as in solid modeling, I would get
Creo Elements/Direct Modeling Express 4.0. Of all the free 3D modeling software available this offering from PTC has got to be the most sophisticated and robust while still easy to learn.
Having said all of that, if I were you I'd call the legal department for Autodesk and ask them what exactly do they consider commercial. The reason I say that is, I could see them wanting you to not use their educational software version to make a fortune, but if one is only gonna scratch together enough money to barely afford the software then why would they tell you to stop using it? They lose nothing by keeping you as a future customer, while if they tell you to stop, you're just going to get familiar with a competitors software and when you do start making enough to afford it why would you relearn their software? You see, with software, they lose nothing if one can't ever afford to buy it, cuz it is a non-tangible item. This makes me think they would have some leeway in their policies. Also too, educational institutions MAKE MONEY in enumerable ways. How else are they to teach students how the real world is? So I'd give them a call and see what they have to say exactly.