Yeah, the problem is that you won't really know if you have a frame rust issue until you pull the windshield. You didn't say anything about the existing windshield having a crack, just the water leak. Pulling the windshield to check the rust is a concern, as the act of removing it might cause it to crack (rare, but does happen.) And of course, once it's removed you are non-drivable. So you need to be sure wherever you take it they can handle both the glass (if breaks) and the rust, as its not going anywhere once they start the work. (Trust me, I know...)
Have you eliminated all other possibilities? Water from leaks migrate downward. Have you checked the seals around your marker lights, roof seams etc? Pulling the windshield would be a last resort after all other spots are checked.
