Surely it depends on the price as to whether the car is worth investing in.
The rust looks pretty bad to me, frankly. I've seen a few cars where a little rust turned out to be quite an awful lot of rust, but sadly these are the discoveries made once one starts to dig. So I'd suggest you expect a lot worse, perhaps. You should assume new sills, I think. And take a good look at the front cross member and around the firewall. You should also assume the underside to be in quite a state. J.Huber's advice is well worth following in detail.
Not sure what the asking price is, but a thorough search of this site will demonstrate that some cars that appear to have a few rust issues are actually in a very poor state. I'd get amongst the search engine if I were you and learn from those who've gone before you.
Do you have an idea of how much the owner wants?