How much can I do on a the air?

I am thinking about selling my ipad 3 and getting an air for school. The tough part about going through with this is how much of my work I can actually do on the air. Im an mechanical engineering student and get a free copy of windows 7 for any software that needs it. So I was wondering if any of you guys have used Catia or Autocad or another other high resources program on the air? And which configuration of the air would be the best for around 1300?

Thanks