The Rockhopper is a fine bike and something I'd have suggested off-the-cuff too but I would really recommend you head to a LBS or a few LBSes and do some testrides. See how they ride. See how other bikes ride. Building up a bike is going to be more expensive because bike manufacturers get deep discounts from the component manufacturers and so they can offer a bike with components to the consumer for far less than you could accomplish with a frame-up build and parts kit.