So I'm not completely new to multiboxing but I think I've already made a mistake or two.

My current setup consists of my level 80 Disc Priest (mostly tier 9.5 or higher items) and my level 80 Prot Paladin (mostly tier 9). I also hope to add a 3rd account in the near future.

I have also purchased a copy of ********, it does however seem that most people here use ISBoxer.

To date I have only been running around and getting some of the old world achievements and would now like to try for something more adverturous. I have some nagging questions however:

1) Would it be better for me to get a copy of ISBoxer instead of ********?
2) Is there any benefit in moving from a 2 box setup to 3 box? or would the only benefit be going from 2 to 5?
3) If there is a benefit in going to 3 box, which character class should I add?
4) What would the best way be to level my new character up quickly considering the 2 I already have?
5) As indicated above, my current setup is a Disc Priest and a Prot Paladin, would I change this if I add a 3rd character?
6) Any additional advice would be greatly appreciated.