2003 GMC Sierra 2500HD

I've found this 2003 2500 HD Id like to buy(because it's at a huge discount) but it's been wrecked on the side and I'm curious if anybody has and ideas on how I can fix the damages. I figured I could get a new door from someone's part out but I'm not sure what I'd do to get the dent out of the roof any help would be great thanks.