Replacing the harddrive in iMac is easy
Follow the iFixit guide for the iMac. You'll need Torx T10, T8, and T6 screwdrivers, a Philips #1 screwdriver, a flat-head screwdriver (or a strong plastic/acrylic spatula that doesn't scratch) and two tile suction cups or a glass/window suction cup grip.
The glass front does lift off fairly easy if you use the suction cups. Do *NOT* try to lever it out with a screwdriver or something similar. I recently used the iFixit guide to replace the optical drive in my iMac and it was done in 25 minutes flat.