Radiator top tanks crack around the neck and cause other issues if ignored. Headgaskets leak coolant and/or oil in a lot of cases. It can be minor and not get any worse for thousands of kays, but if it's pissing out and making all the rubber stuff around it go soft you better do them.
Generally rust is non existent unless they have been accident repaired. Look for heater core leaks or by-pass as it's expensive to repair (although not any more common than others)
If it's metallic paint look for blisters in the clear coat or evidence of cheapo re-coating as that's also common.