I have a friend that welds for a living, and has been welding cars for quite some time. He had a look at it today, and the first thing he did was laugh, but at the same time he said it'll be easy enough, the sills will be replaced entirely, not just filled in. Old sills out, new sills in. Also before the paint job commences and other bits of rust will be sorted also in the fashion of cut out and weld in fresh metal.
Car will be off the road from the time it gets welding treatment until it is finished with new paint.