Kidcreation, assuming you have not yet paid any money as deposit or outright purchase, I would make the following observations:
1. Whilst that rust could be removed, treated & painted, it looks very much like the surrounding sills and probably other areas have already been re-sprayed. If so, rust has re-appeared and that makes me doubt that the panels were properly prepared before that (cosmetic??) re-spray. If I were considering that car, I would insist on taking a good look underneath, to see what other rust is visible. The front mounts of both the front & rear sub-frames have a reputation for rotting first. There is also a notorious mud-trap at the back of the front wheel arches. Water & muck drop down the wheel-arch liner, and are trapped between plastic & steel. Rot follows.
2. How much service history, invoices or (ideally) a photo history of work being done. Cambelt changes and a properly replaced head gasket are the two "must haves" in my mind, unless the car is cheap.

Back in March 2021, I found rust in the same rear sill area as you. Photos of the rust spots and the re-spray are attached. At least your paint colour will be easier to match than my Starlight Silver!

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