Possible yes, but of course you'll have to manage your storage very carefully to not run out of space. If you have not done it already, try using
Monolingual to remove languages you don't use from OS X and save space.
You can download Windows 7 ISOs legally directly from Microsoft's servers. Links are
here. You will of course need a valid serial number for activation, which you can get when you buy a boxed copy of Windows 7.