Most assignments will usually be printed out and handed in, so it won't matter what program they are typed on. However, you may have to accept you will sometimes need to use the university computers to download attatchments, or if you are doing group work and everyone else is using microsoft office. It would probably be best to prepare presentations on powerpoint too.
If you are looking for something less expensive, what about looking at netbooks instead of laptops? They also have the advantage of being smaller and easier to carry.