Bottom line, it's going to be a difficult, time consuming, and costly repair that will take special tools and knowledge. You're going to need to fabricate panels, weld, cut, paint, etc...And you need the tools and knowledge to do all of the above.
As far as structural integrity, that's not exactly part of the unibody support structure, but it's not helping the structural integrity of the car overall either. I'd highly recommend fixing it before seriously starting to use the car for any sort of driving.
Check out this thread:
click
Scroll down to my post where I link to 3 threads. Your rust repair will fall in between "Bobdole's rust repair" thread and the "MAJOR rust repair" thread.
The last thing to consider is the cost of a repair like this. Even doing it yourself, you'll be into the job $500-$1,000 bucks before you know it.
Edit: I reference using a small Campbell Hausfeld 110v gasless MIG welder to do repairs in my post. Although this worked fine on 1837 for a very small repair, it was complete sh*t for the larger repair I had to do on 820. In short, you'll need a legit MIG/TIG welder, and someone who knows how to use it for a job like yours.