hello...
well, mine were ONLY the outer sills. Once we cut away the rust to good metal, we could inspect the inner sill, which was solid. We used a special fibreglass repair kit to build up the sill, and then a metal compound to finish. We got the sill looking really good, and solid, but I also drilled two drainage holes per sill, to ensure water is draining properly from inside the sill.
I think if you find the inner sill rusty and thin, get it welded, I dropped lucky with mine. I also treated the inner sill, and outer sill when complete, with kurust then Hammerite direct to rust paint, whcih also turns any rust solid, so handy stuff. MOT man was happy with it, so it passed.
Might not be so lucky in a year or so, but we will see. I think its a good idea to get the carpets up in the rear to inspect the seat belt mounting area, and inspect via the rear wheel arch, can giev you a good indication of what to expect from your inner sills....