I have the same rust on my bonnet and tailgate, and the body shell and chassis are perfectly solid. The offside front wheel arch needed a plate on the rear slope, as did the foot well, but the rest is completely sound (I have a mechanic friend with a boroscope and we checked all the cavities, sills, inner wings and so on).
Localised media blasting and lead loading would be the best way of dealing with the pictured rust, which is almost certainly surface rust. If it does go through, then plating on the inside and filling would work perfectly well.