The RPM definitely influences the edit. Considering how inexpensive hard drives are, I don't recommend anything less than 7200RPM.
Also, wouldn't the direct connection of an internal hard drive be faster than the USB2 connection for an external?
Personally, I think external is best only for backup, and internal is what you edit on. You can get a TB for less than $100, and I don't think they even make 5400RPM internal drives any more. Of course I have no clue how easy or difficult it is for you to get stuff shipped to you, in Bangladesh.