my 2007 Tahoe LTZ came with rear DVD but no Nav. I want to add Nav to my Truck.....should I go with after market or with factory......is adding a factory system difficult?

I found this system on ebay......would it be as simple as unplugging my current unit and plugging this one in? will i retain all my functions? ie steering wheel controls, XM radio, rear DVD?

http://cgi.ebay.com/ebaymotors/ws/eB...TQ:MOTORS:1123

I could also purchase a new one for about 2x the price:

http://www.gmpartsdirect.com/results...umber=19153969

If difficult to instal on my own what would GM dealership charge to swap out?