If it was only the typical rust that can surface in the rear fenders I'd say go for it, yet it would still likely be $4k+ to fix. However with all of the area's you mentioned I would say it's definitely not worth the excessive repair costs. The fact that it's visible along the side skirts is some real insight to how far gone it could be, or will be in a short time. It would be safe to assume that the bottom rear of the front fenders, the bottom front of the rear fenders and possibly other spots behind the side skirts could be rotting pretty bad. I've experienced this on my high mileage 02 WRX wagon. No noticeable rust except when pulling the side skirt, and not a pleasant surprise. The car was a Calgary car since new, until I bought it.
With the mention of it sitting for years, I can say that definitely doesn't help out with corrosion. If anything it makes it worse with humidity, precipitation and sitting stagnant not getting any air flowing through things (when driving).