I hate to say it, but that's some of the worst rust I've seen on a 100. Has the moulding around the windshield been removed? I'm wondering if the windshield was replaced, the moulding was put back in poorly or wrong and water was allowed to get trapped back there. That's all hindsight tho, doesn't really help you now.
Looks like it's eaten clear through in a few spots. Cutting that lip out and replacing it certainly could run into thousands for labor, body work, and paint. Cheap solution would be to get the rust out of there (flapper wheel on a drill, dremel, grinder with sanding disc, etc.), coat everything with a rust stopping paint like POR15 or similar, fill the holes with body filler or silicon, and then make sure that the OEM moulding is replaced and sealed correctly.