The differences between getting n poly and outside is:
Getting in poly is cheaper, getting from outside you can customise
the specification.
About software and stuffs, the school will provide you whether you
buy your laptop from sch or outside.
If buy from sch, the software is already in it. If buy from
outside, you have to waste a bit of your time for the sch to
install the softwares in your laptop.
If window vista, the sch will re-install the whole laptop to win
xp. That is how much poly hate vista. 
Total installation time can be up to 6 hrs, be prepared.
