1. Yes, a case fan can be used.
2. The fan should be blowing in the same direction as the current fan that is on there.
3. New egg has a great power search function, i suggest you use this to choose your specifics. A fan that is >90cfm and <25 dba will suffice. Make sure you are either buying a fan that is equal or more powerful than the current fan, otherwise you will have increased temps.
4. Yes, connect it directly into the PSU, or one of the 3 pin power headers oN the mobo.
Hope that helps...