The outer sills are only covers and not part of the structural integrity of the car as has already been mentioned.
Although Althulus has pointed out an extreme case of inner sill corrosion, in all the years of owning Z3MCs since 2005 I’ve only been aware of two cases of inner sills requiring repair, one on Z Coupe Forum and if I recall one car on this Forum. Other than that everyone who has carried out refurbishment of inner sills seems to have done it along the lines of how I did mine.
Looking at the car you have in mind (bearing in mind we are looking at photos) it generally looks in good condition.
What is required are either close up photos of corrosion around jacking points or a physical inspection of the car to see if it’s just corroded sill covers. Without removing outer covers you cannot see the condition of inner sills.
Hope this helps.