I have enough money to buy the top config 27" 2013 model and the employee discount I want to get the most basic 21.5" model....Should I get it? I plan on using it for internet, iWork, and music and video editing. For the music/video editing is this good enough or should I go as high as possible, maybe even buying a Mac Pro?
I do professional video editing on my 2013 iMac 27 i7@3.5Ghz, 32GB RAM, GTX-780M and 3TB Fusion Drive. It works fine.
The 21.5" is capable also, your main issue is physical size. The first impression is the 27 is big and over-large for many desks and work areas. My wife almost got a 2012 21.5" for that reason -- she has a small desk (39" wide and 24" deep).
She finally got a 2012 27", which fit just fine and looks great. It is so thin and aesthetically well designed it visually blends in much better than the 27" size would indicate. The extra screen real estate and resolution is very useful.
While video editing is possible on a much smaller screen or a laptop, the extra screen real estate is handy, since you often have multiple windows open.
Re Fusion Drive vs SSD, I am happy with FD but SSD is faster -- provided all your material fits on it. Get either FD or SSD, don't get a plain HDD.
Re performance in FCP X, the 2013 iMacs have a Haswell CPU with Quick Sync instructions (which FCP uses). This allows high-speed, hardware-assisted rendering for certain commonly-used codecs. Exporting to single-pass 1080p H.264, you might see 5x performance increase (or more) over your current system. Other export types using Compressor will not be similarly accelerated but the most commonly-used ones are.