It is not at all difficult to install Windows 7. Once you have downloaded it all you have to do is burn the .iso file to a disc.
Install it and use the Blue Badge removal tool under tutorials and bob's your uncle.
It runs extremely smoothly for a pre-beta, but if you have any concerns, wait for the Beta.
And as for the hard drive advice, just buy whatever will fit in with your set up, it does not have to be a huge hard drive. But if you have the space on your hard drive why not dual boot?