The roof piece doesn't look too worrying to me. Looks like someone put lots of bondo over the whole car, and that piece started lifting as a result of the surface rust you see there. You'll need to treat the surface rust, but it doesn't look serious at all - at least from what we can see. I would remove the bondo and make certain there's no unseen issues.
As for your fender, I would continue to remove the bondo there, and see what exactly is going on there. The welds make me think it's someone's attempt at making a straight piece of sheet into a curved piece by cutting pie cuts and welding them up. It also appears to be the inner part of the fender, not the outer part, which appears to have been cut off, and left as is. I'm thinking the fender lips were either rusted, or cut off for fender flares which were later removed. I think the easiest way of fixing this, assuming I'm right, is to purchase an inner wheel well, and partial outer quarter, and weld those in. You should also check out the other side since, whether it's as a result or rust or old fender flares, both sides probably have work done to them. I'd start by looking at the inner wheel well, no sense removing paint if you don't need to.