I just got back from an oil change at the dealer - $75 with a coupon! - and he told me the serpentine belt tensioner need to be replaced. His price is $445 or so.

Is the a DIY job? Do I need a special tool, if so, does anyone rent it? My X5 is a 2009 3.0. I've seen the You Tube videos but they are all kinda generic.
Crazy thing is I have a brand new fan tool for the E53 I no longer have. I bought it just for the same job on that car.