I would buy a set of new sills but then cut out sections of it to replace the rusty bits. It will always rust in front of the rear wheels and behind the front wheels.
I would replace about 30 cms at each end with the cut off sill sections.
I fabricated replacement sections but I'm going to redo it with cut down sills next time. This is where I repaired it the first time-

Fixing sill panel rust

It won't be how long it's taking the guy to fix it. Presumably he's just doing a bit here and a bit there between other jobs.
You can't rush perfection!