BB, There is a version of the MS Office suite for OS X. If you are going Mac, that should be your first option. Why not just run Word, Excel and PP right on your Mac within OS X?
If you do want to run Windows (or another operating system) on your iMac, though, you don't need Parallels necessarily, unless you want to be able to boot Windows within OS X.
If you are fine booting one operating system at a time to use whatever programs you want on that OS, you use "Boot Camp," which is built into OS X, to add another operating system, such as Windows. When your iMac is booting, hold down the option key and it will ask you to choose which operating system you want it to boot into. You can choose OS X or Windows (or whatever other operating systems you have sitting on a disk partition).
If you decide you want to be able to open Windows programs from within OS X, Parallels *is* worth it. I run it on several machines.