Entirely depends on what sort of cooling you're using - air or water, and if air then how good is the airflow through your case? And when you say average, do you mean the average for load or idle?
Here's my best guess, using the no information given. These are in celcius, of course.
1) No more than 70 on load, no more than 40 idle.
2) Assuming stock air cooling, no more than 45 idle and no more than 80 load.
3) ....system?
4) About 30-35, hard drives don't really get that hot unless there's hours of constant full use.