Have you ever wondered how difficult it might be to switch Sage from Python 2.7 to Python 3.x? Some students in my Sage course made a webpage that summarizes the Python 3 support status of the Python packages on which Sage depends.
Matplotlib doesn't support Python 3.x at all yet.
Mercurial doesn't support Python 3.x, and they don't have any plans to do so. (Which is another point in favor switching Sage to GIT.)
If your calculation will take less than 20 seconds (?), you can do it over the web for free. (Historical note: I wrote the first version of that website.)
In Sage, use the command magma_free('some string'), though I just checked and the Magma calculator has changed in a way that breaks this command again. I'll be posting a patch to fix this in Sage.
Most people I know who have Magma do not buy it, but get it in exchange for contributing to Magma (I used to get it free for that reason). Study the list of past and current contributors to Magma and ask the one you know the best. This is a list of people who definitely have a copy of Magma and know how to use it.