computer upgrade for a new teacher

hi all- i just started a position as a lazy teacher and would like to upgrade either/or my 2006 pc laptop or my 2006 classroom dell desktop. I don't need to do both... but I have an iMac and iphone and will probably end up using an ipad in there someday.

In my free time i mess around with ableton live and some photo editing programs, but my understanding is that an MBA can pretty much handle this at this point. my question is: what should I get?

I realize it is kind of loaded. I've thought of going for an mba, a used mac mini that i would hope can work with the monitor at my school... or even replacing my school desktop with my old laptop (would be an upgrade) so i can go dual display when i'm drowning in data collection spreadsheets. what do people think of that one?