If you are not used top cossies, my advice, take someone who is, especially for the test drive.
Rust will be evident due to age of car, but other damage such as mentioned accident bodged repairs can be very costly to put right, so must be reflected in price.
Rear arches are not going to leave you any change out of £500.
I wouldn't worry about original dated wings, as if they are from 1986 etc you know that rust is going to be under the pain in places.
Accident damage is more likely to be a problem that is harder to spot, which is why it would be best to take someone with you who knows about these, even if you have to bung them a few quid.
tabetha